cmd/compile: log Ctz non-zero proofs

I forgot this in CL 109358.

Change-Id: Ia5e8bd9cf43393f098b101a0d6a0c526e3e4f101
Reviewed-on: https://go-review.googlesource.com/109775
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -774,6 +774,9 @@ func simplifyBlock(sdom SparseTree, ft *factsTable, b *Block) {
 				continue
 			}
 			if lim.umin > 0 || lim.min > 0 || lim.max < 0 {
+				if b.Func.pass.debug > 0 {
+					b.Func.Warnl(v.Pos, "Proved %v non-zero", v.Op)
+				}
 				v.Op = ctzNonZeroOp[v.Op]
 			}
 		}