Revert "Treat blocks with "return" as "noreturn""

This reverts commit 135b91aecd9be1f6f5806b667e07e383dd481198.

"endsWithBreak()" is not a meaningful question to ask and should not be
used this way. A simple example that defeats this logic is:

```zig
export fn entry() void {
    outer: {
        {
            break :outer;
        }
        return;
    }
}
```

src/AstGen.zig

@@ -1967,9 +1967,6 @@ fn blockExpr(
     }
 
     try blockExprStmts(gz, scope, statements);
-    if (gz.endsWithNoReturn() and !gz.endsWithBreak()) {
-        return Zir.Inst.Ref.unreachable_value;
-    }
     return rvalue(gz, rl, .void_value, block_node);
 }
 
@@ -9933,13 +9930,6 @@ const GenZir = struct {
         return tags[last_inst].isNoReturn();
     }
 
-    fn endsWithBreak(gz: GenZir) bool {
-        if (gz.isEmpty()) return false;
-        const tags = gz.astgen.instructions.items(.tag);
-        const last_inst = gz.instructions.items[gz.instructions.items.len - 1];
-        return tags[last_inst].isBreak();
-    }
-
     /// TODO all uses of this should be replaced with uses of `endsWithNoReturn`.
     fn refIsNoReturn(gz: GenZir, inst_ref: Zir.Inst.Ref) bool {
         if (inst_ref == .unreachable_value) return true;

src/Zir.zig

@@ -1250,19 +1250,6 @@ pub const Inst = struct {
             };
         }
 
-        /// Returns whether the instruction is a "break". This differs from
-        /// isNoReturn because a "break" in a block statement is not a
-        /// "noreturn" for the outer scope, whereas the other "noreturn"
-        /// instructions are.
-        pub fn isBreak(tag: Tag) bool {
-            return switch (tag) {
-                .@"break",
-                .break_inline,
-                => true,
-                else => false,
-            };
-        }
-
         /// AstGen uses this to find out if `Ref.void_value` should be used in place
         /// of the result of a given instruction. This allows Sema to forego adding
         /// the instruction to the map after analysis.

test/cases/compile_errors/stage2/code_after_return_in_block_is_unreachable.zig

@@ -1,14 +0,0 @@
-export fn entry() void {
-    {
-        return;
-    }
-
-    return;
-}
-
-// error
-// target=native
-//
-// :6:5: error: unreachable code
-// :2:5: note: control flow is diverted here
-