Ensure stack allocations maintain 16-byte alignment for AVX instructions

src/xenia/cpu/backend/x64/x64_emitter.cc

@@ -231,7 +231,13 @@ bool X64Emitter::Emit(HIRBuilder* builder, EmitFunctionInfo& func_info) {
     locals_page_delta_ = 0;
   }
 
-  assert_true((stack_size + 8) % 16 == 0);
+  // Ensure that after we SUB the stack_size, RSP is 16-byte aligned.
+  // On function entry, RSP is misaligned by 8 (due to CALL pushing return
+  // address). So we need stack_size to be 8 mod 16, not 0 mod 16.
+  if ((stack_size % 16) == 0) {
+    stack_size += 8;  // Make it 8 mod 16
+  }
+  assert_true((stack_size % 16) == 8);  // Should be 8 mod 16 for alignment
   func_info.stack_size = stack_size;
   stack_size_ = stack_size;