cmd/compile: add test for 42753

This issue was already fixed at tip. Just adding the test that
failed on 1.14/1.15.

Update #42753

Change-Id: I00d13ade476b9c17190d762d7fdcb30cf6c83954
Reviewed-on: https://go-review.googlesource.com/c/go/+/272029
Trust: Keith Randall <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: David Chase <[email protected]>

Author: randall77

test/fixedbugs/issue42753.go

@@ -0,0 +1,13 @@
+// compile -d=ssa/check/on
+
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func f() uint32 {
+	s := "\x01"
+	x := -int32(s[0])
+	return uint32(x) & 0x7fffffff
+}