Sema: avoid emitting loops that can't loop

If a `loop` ends with a `noreturn` instruction, then it cannot loop and
will be emitted as a `block` instead.

src/Air.zig

@@ -221,11 +221,16 @@ pub const Inst = struct {
         /// Reinterpret the memory representation of a value as a different type.
         /// Uses the `ty_op` field.
         bitcast,
-        /// Uses the `ty_pl` field with payload `Block`.
+        /// Uses the `ty_pl` field with payload `Block`.  A block runs its body which always ends
+        /// with a `noreturn` instruction, so the only way to proceed to the code after the `block`
+        /// is to encounter a `br` that targets this `block`.  If the `block` type is `noreturn`,
+        /// then there do not exist any `br` instructions targetting this `block`.
         block,
         /// A labeled block of code that loops forever. At the end of the body it is implied
         /// to repeat; no explicit "repeat" instruction terminates loop bodies.
-        /// Result type is always noreturn; no instructions in a block follow this one.
+        /// Result type is always `noreturn`; no instructions in a block follow this one.
+        /// The body never ends with a `noreturn` instruction, so the "repeat" operation
+        /// is always statically reachable.
         /// Uses the `ty_pl` field. Payload is `Block`.
         loop,
         /// Return from a block with a result.

src/Sema.zig

@@ -5294,14 +5294,20 @@ fn zirLoop(sema: *Sema, parent_block: *Block, inst: Zir.Inst.Index) CompileError
 
     try sema.analyzeBody(&loop_block, body);
 
-    try child_block.instructions.append(gpa, loop_inst);
+    if (sema.typeOf(Air.indexToRef(loop_block.instructions.items[loop_block.instructions.items.len - 1])).isNoReturn()) {
+        // If the loop ended with a noreturn terminator, then there is no way for it to loop,
+        // so we can just use the block instead.
+        try child_block.instructions.appendSlice(gpa, loop_block.instructions.items);
+    } else {
+        try child_block.instructions.append(gpa, loop_inst);
 
-    try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.Block).Struct.fields.len +
-        loop_block.instructions.items.len);
-    sema.air_instructions.items(.data)[loop_inst].ty_pl.payload = sema.addExtraAssumeCapacity(
-        Air.Block{ .body_len = @intCast(u32, loop_block.instructions.items.len) },
-    );
-    sema.air_extra.appendSliceAssumeCapacity(loop_block.instructions.items);
+        try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.Block).Struct.fields.len +
+            loop_block.instructions.items.len);
+        sema.air_instructions.items(.data)[loop_inst].ty_pl.payload = sema.addExtraAssumeCapacity(
+            Air.Block{ .body_len = @intCast(u32, loop_block.instructions.items.len) },
+        );
+        sema.air_extra.appendSliceAssumeCapacity(loop_block.instructions.items);
+    }
     return sema.analyzeBlockBody(parent_block, src, &child_block, merges);
 }