cmd/compile: use AuxInt to store shift boundedness

Fixes ssacheck build.

Change-Id: Idf1d2ea9a971a1f17f2fca568099e870bb5d913f
Reviewed-on: https://go-review.googlesource.com/110122
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

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

@@ -994,7 +994,7 @@ func simplifyBlock(sdom SparseTree, ft *factsTable, b *Block) {
 			}
 			bits := 8 * v.Args[0].Type.Size()
 			if lim.umax < uint64(bits) || (lim.max < bits && ft.isNonNegative(by)) {
-				v.Aux = true
+				v.AuxInt = 1 // see shiftIsBounded
 				if b.Func.pass.debug > 0 {
 					b.Func.Warnl(v.Pos, "Proved %v bounded", v.Op)
 				}

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

@@ -393,7 +393,7 @@ func b2i(b bool) int64 {
 // shiftIsBounded reports whether (left/right) shift Value v is known to be bounded.
 // A shift is bounded if it is shifting by less than the width of the shifted value.
 func shiftIsBounded(v *Value) bool {
-	return v.Aux != nil && v.Aux.(bool)
+	return v.AuxInt != 0
 }
 
 // i2f is used in rules for converting from an AuxInt to a float.