cmd/compile: add valState to ssa sizeof tests

I learned from CL 41770 that valState’s size
matters to compiler performance.
Encode that knowledge in a test.

Change-Id: I7c0fde6a4cf460017619dbcce1c1ddaa7af10239
Reviewed-on: https://go-review.googlesource.com/44737
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -24,6 +24,7 @@ func TestSizeof(t *testing.T) {
 	}{
 		{Value{}, 68, 112},
 		{Block{}, 152, 288},
+		{valState{}, 28, 40},
 	}
 
 	for _, tt := range tests {