cmd/compile: compute XOR's limits from argument's limits
This help to optimize code like this:
func f(buckets *[512]bucket, v value) {
a, b := v.computeSomething()
// assume a and b are proved < 512
b := &buckets[a ^ b] // pick a random bucket
b.store(v)
}
Change-Id: I1acf702f5a8137f9ded49081b4703922879b0288
Reviewed-on: https://go-review.googlesource.com/c/go/+/604455 Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>