[dev.ssa] cmd/compile/ssa : fix typo in error message

Change-Id: Ibb5169aade15190773ff7dd11b303c1f1345a0c2
Reviewed-on: https://go-review.googlesource.com/13100Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -351,7 +351,7 @@ func regalloc(f *Func) {
 		if b.Kind == BlockCall {
 			call = b.Control
 			if call != b.Values[len(b.Values)-1] {
-				b.Fatalf("call not at end of block %b %v", b, call)
+				b.Fatalf("call not at end of block %v %v", b, call)
 			}
 			b.Values = b.Values[:len(b.Values)-1]
 			// TODO: do this for all control types?