cmd/compile: derive bounds on signed %N for N a power of 2

cmd/compile: derive bounds on signed %N for N a power of 2

-N+1 <= x % N <= N-1

This is useful for cases like:

func setBit(b []byte, i int) {
    b[i/8] |= 1<<(i%8)
}

The shift does not need protection against larger-than-7 cases.
(It does still need protection against <0 cases.)

Change-Id: Idf83101386af538548bfeb6e2928cea855610ce2
Reviewed-on: https://go-review.googlesource.com/c/go/+/672995
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>