Sema: improve source location after as_node is used

+2 more passing compile error tests

src/Sema.zig

@@ -8321,6 +8321,7 @@ fn zirAsNode(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air.
     const inst_data = sema.code.instructions.items(.data)[inst].pl_node;
     const src = inst_data.src();
     const extra = sema.code.extraData(Zir.Inst.As, inst_data.payload_index).data;
+    sema.src = src;
     return sema.analyzeAs(block, src, extra.dest_type, extra.operand, false);
 }
 

test/cases/compile_errors/and_on_undefined_value.zig

@@ -0,0 +1,10 @@
+comptime {
+    var a: bool = undefined;
+    _ = a and a;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:9: error: use of undefined value here causes undefined behavior

test/cases/compile_errors/or_on_undefined_value.zig

@@ -0,0 +1,10 @@
+comptime {
+    var a: bool = undefined;
+    _ = a or a;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:9: error: use of undefined value here causes undefined behavior

test/cases/compile_errors/stage1/and_on_undefined_value.zig

@@ -1,10 +0,0 @@
-comptime {
-    var a: bool = undefined;
-    _ = a and a;
-}
-
-// error
-// backend=stage1
-// target=native
-//
-// tmp.zig:3:9: error: use of undefined value here causes undefined behavior

test/cases/compile_errors/stage1/obj/reify_type_for_union_with_opaque_field.zig

@@ -8,8 +8,8 @@ const Untagged = @Type(.{
         .decls = &.{},
     },
 });
-export fn entry() void {
-    _ = Untagged{};
+export fn entry() usize {
+    return @sizeOf(Untagged);
 }
 
 // error

test/cases/compile_errors/stage1/or_on_undefined_value.zig

@@ -1,10 +0,0 @@
-comptime {
-    var a: bool = undefined;
-    _ = a or a;
-}
-
-// error
-// backend=stage1
-// target=native
-//
-// tmp.zig:3:9: error: use of undefined value here causes undefined behavior