cmd/compile: mask SLL,SRL,SRAconst shift amount

mips SRA/SLL/SRL shift amounts are used mod 32; this change aligns the
XXXconst rules to mask the shift amount by &31.

Passes

  $ GOARCH=mips go build -toolexec 'toolstash -cmp' -a std
  $ GOARCH=mipsle go build -toolexec 'toolstash -cmp' -a std

Fixes #42587

Change-Id: I6003ebd0bc500fba4cf6fb10254e1b557bf8c48f
Reviewed-on: https://go-review.googlesource.com/c/go/+/270117
Trust: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/test/fixedbugs/issue42587.go b/test/fixedbugs/issue42587.go
new file mode 100644
index 0000000000..d10ba979d5
--- /dev/null
+++ b/test/fixedbugs/issue42587.go
@@ -0,0 +1,15 @@
+// compile
+
+// 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 p
+
+func f() {
+	var i, j int
+	_ = func() {
+		i = 32
+		j = j>>i | len([]int{})
+	}
+}