cmd/compile: loop in disjoint OpOffPtr check

We collapse OpOffPtrs during generic rewrites.
However, we also use disjoint at the same time.
Instead of waiting for all OpOffPtrs to be collapsed
before the disjointness rules can kick in,
burrow through all OpOffPtrs immediately.

Change-Id: I60d0a70a9b4605b1817db7c4aab0c0d789651c90
Reviewed-on: https://go-review.googlesource.com/c/145206
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Michael Munday <mike.munday@ibm.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/rewrite.go

@@ -542,7 +542,7 @@ func disjoint(p1 *Value, n1 int64, p2 *Value, n2 int64) bool {
 	}
 	baseAndOffset := func(ptr *Value) (base *Value, offset int64) {
 		base, offset = ptr, 0
-		if base.Op == OpOffPtr {
+		for base.Op == OpOffPtr {
 			offset += base.AuxInt
 			base = base.Args[0]
 		}