fix peer result locs with switch

BRANCH_TODO

@@ -1,6 +1,8 @@
 Scratch pad for stuff to do before merging master
 =================================================
 
+uncomment all the behavior tests
+
 restore test_runner.zig to master branch
  - also the default panic function and unexpected_error_tracing. see the commit
    that adds this text to BRANCH_TODO file.
@@ -8,11 +10,21 @@ restore test_runner.zig to master branch
 
 get an empty file compiling successfully (with no panic fn override)
 
-uncomment all the behavior tests
-
 better behavior for implicit casts. for example these introduce an extra allocation/memcpy:
     var x: [1]i32 = [_]i32{1};
     var x = ([1]i32)([_]i32{1});
 whereas this one does not:
     var x = [_]i32{1};
 but all 3 should be semantically identical
+
+
+This example has less than ideal LLVM IR:
+```zig
+export fn entry() void {
+    _ = mul(true, 1) catch undefined;
+}
+pub fn mul(c: bool, answer: i32) error{Overflow}!i32 {
+    return if (c) error.Overflow else answer;
+}
+```
+It creates an unnecessary stack variable.

src/all_types.hpp

@@ -3662,6 +3662,7 @@ struct ResultLocPeerParent {
 
     bool skipped;
     bool done_resuming;
+    IrBasicBlock *end_bb;
     ResultLoc *parent;
     ResultLocPeer *peers;
     size_t peer_count;

src/ir.cpp

@@ -3901,6 +3901,7 @@ static ResultLocPeerParent *create_binary_result_peers(IrInstruction *cond_br_in
     ResultLocPeerParent *peer_parent = allocate<ResultLocPeerParent>(1);
     peer_parent->base.id = ResultLocIdPeerParent;
     peer_parent->base.source_instruction = cond_br_inst;
+    peer_parent->end_bb = endif_block;
     peer_parent->is_comptime = is_comptime;
     peer_parent->parent = parent;
     peer_parent->peer_count = 2;
@@ -6894,6 +6895,7 @@ static IrInstruction *ir_gen_switch_expr(IrBuilder *irb, Scope *scope, AstNode *
 
     ResultLocPeerParent *peer_parent = allocate<ResultLocPeerParent>(1);
     peer_parent->base.id = ResultLocIdPeerParent;
+    peer_parent->end_bb = end_block;
     peer_parent->is_comptime = is_comptime;
     peer_parent->parent = result_loc;
     peer_parent->peers = allocate<ResultLocPeer>(prong_count);
@@ -14923,9 +14925,8 @@ static IrInstruction *ir_resolve_result_raw(IrAnalyze *ira, IrInstruction *suspe
             }
 
             if (peer_parent->resolved_type == nullptr) {
-                ResultLocPeer *last_peer = &peer_parent->peers[peer_parent->peer_count - 1];
-                if (last_peer->next_bb->suspend_instruction_ref == nullptr) {
-                    last_peer->next_bb->suspend_instruction_ref = suspend_source_instr;
+                if (peer_parent->end_bb->suspend_instruction_ref == nullptr) {
+                    peer_parent->end_bb->suspend_instruction_ref = suspend_source_instr;
                 }
                 return ira_suspend(ira, suspend_source_instr, result_peer->next_bb, &result_peer->suspend_pos);
             }

test/stage1/behavior.zig

@@ -31,7 +31,7 @@ comptime {
     _ = @import("behavior/bugs/421.zig");
     _ = @import("behavior/bugs/529.zig");
     _ = @import("behavior/bugs/655.zig");
-    //_ = @import("behavior/bugs/656.zig");
+    _ = @import("behavior/bugs/656.zig");
     _ = @import("behavior/bugs/679.zig");
     _ = @import("behavior/bugs/704.zig");
     _ = @import("behavior/bugs/718.zig");
@@ -71,7 +71,7 @@ comptime {
     _ = @import("behavior/popcount.zig");
     //_ = @import("behavior/ptrcast.zig");
     _ = @import("behavior/pub_enum.zig");
-    //_ = @import("behavior/ref_var_in_if_after_if_2nd_switch_prong.zig");
+    _ = @import("behavior/ref_var_in_if_after_if_2nd_switch_prong.zig");
     _ = @import("behavior/reflection.zig");
     _ = @import("behavior/sizeof_and_typeof.zig");
     //_ = @import("behavior/slice.zig");
@@ -81,7 +81,7 @@ comptime {
     _ = @import("behavior/struct_contains_slice_of_itself.zig");
     //_ = @import("behavior/switch.zig");
     //_ = @import("behavior/switch_prong_err_enum.zig");
-    //_ = @import("behavior/switch_prong_implicit_cast.zig");
+    _ = @import("behavior/switch_prong_implicit_cast.zig");
     _ = @import("behavior/syntax.zig");
     _ = @import("behavior/this.zig");
     _ = @import("behavior/truncate.zig");
@@ -90,7 +90,7 @@ comptime {
     //_ = @import("behavior/typename.zig");
     _ = @import("behavior/undefined.zig");
     _ = @import("behavior/underscore.zig");
-    //_ = @import("behavior/union.zig");
+    _ = @import("behavior/union.zig");
     _ = @import("behavior/var_args.zig");
     _ = @import("behavior/vector.zig");
     _ = @import("behavior/void.zig");