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;
    }
}
```

1 files changed, 0 insertions, 13 deletions

diff --git a/src/Zir.zig b/src/Zir.zig
index bad7b91488..f09f2015e0 100644
--- a/src/Zir.zig
+++ b/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.