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 <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

test/fixedbugs/issue42753.go

diff --git a/test/fixedbugs/issue42753.go b/test/fixedbugs/issue42753.go
new file mode 100644
index 0000000..a998d1d
--- /dev/null
+++ b/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
+}