cmd/compile/internal/ssa: refactor zeroUpper32Bits

Explicitly whitelist args of OpSelect{1|2} that zero upper 32 bits.
Use better values in corresponding test.
This should have been a part of  CL 96815, but it was submitted, before
relevant comments.

Change-Id: Ic85d90a4471a17f6d64f8f5c405f21378bf3a30d
Reviewed-on: https://go-review.googlesource.com/97295
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -804,9 +804,9 @@ func zeroUpper32Bits(x *Value, depth int) bool {
 		OpAMD64ANDL, OpAMD64ANDLconst, OpAMD64ORL, OpAMD64ORLconst,
 		OpAMD64XORL, OpAMD64XORLconst, OpAMD64NEGL, OpAMD64NOTL:
 		return true
-	case OpArg, OpSelect0, OpSelect1:
+	case OpArg:
 		return x.Type.Width == 4
-	case OpPhi:
+	case OpPhi, OpSelect0, OpSelect1:
 		// Phis can use each-other as an arguments, instead of tracking visited values,
 		// just limit recursion depth.
 		if depth <= 0 {

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

@@ -17,11 +17,11 @@ type extTest struct {
 var extTests = [...]extTest{
 	{f: func(a, b uint64) uint64 { op1 := int32(a); op2 := int32(b); return uint64(uint32(op1 / op2)) }, arg1: 0x1, arg2: 0xfffffffeffffffff, res: 0xffffffff, name: "div"},
 	{f: func(a, b uint64) uint64 { op1 := int32(a); op2 := int32(b); return uint64(uint32(op1 * op2)) }, arg1: 0x1, arg2: 0x100000001, res: 0x1, name: "mul"},
-	{f: func(a, b uint64) uint64 { op1 := int32(a); op2 := int32(b); return uint64(uint32(op1 + op2)) }, arg1: 0x1, arg2: 0xfffffffffffffff, res: 0x0, name: "add"},
-	{f: func(a, b uint64) uint64 { op1 := int32(a); op2 := int32(b); return uint64(uint32(op1 - op2)) }, arg1: 0x1, arg2: 0xfffffffffffffff, res: 0x2, name: "sub"},
-	{f: func(a, b uint64) uint64 { op1 := int32(a); op2 := int32(b); return uint64(uint32(op1 | op2)) }, arg1: 0x1, arg2: 0xfffffffffffffff, res: 0xffffffff, name: "or"},
-	{f: func(a, b uint64) uint64 { op1 := int32(a); op2 := int32(b); return uint64(uint32(op1 ^ op2)) }, arg1: 0x1, arg2: 0xfffffffffffffff, res: 0xfffffffe, name: "xor"},
-	{f: func(a, b uint64) uint64 { op1 := int32(a); op2 := int32(b); return uint64(uint32(op1 & op2)) }, arg1: 0x1, arg2: 0x100000000000001, res: 0x1, name: "and"},
+	{f: func(a, b uint64) uint64 { op1 := int32(a); op2 := int32(b); return uint64(uint32(op1 + op2)) }, arg1: 0x1, arg2: 0xeeeeeeeeffffffff, res: 0x0, name: "add"},
+	{f: func(a, b uint64) uint64 { op1 := int32(a); op2 := int32(b); return uint64(uint32(op1 - op2)) }, arg1: 0x1, arg2: 0xeeeeeeeeffffffff, res: 0x2, name: "sub"},
+	{f: func(a, b uint64) uint64 { op1 := int32(a); op2 := int32(b); return uint64(uint32(op1 | op2)) }, arg1: 0x100000000000001, arg2: 0xfffffffffffffff, res: 0xffffffff, name: "or"},
+	{f: func(a, b uint64) uint64 { op1 := int32(a); op2 := int32(b); return uint64(uint32(op1 ^ op2)) }, arg1: 0x100000000000001, arg2: 0xfffffffffffffff, res: 0xfffffffe, name: "xor"},
+	{f: func(a, b uint64) uint64 { op1 := int32(a); op2 := int32(b); return uint64(uint32(op1 & op2)) }, arg1: 0x100000000000001, arg2: 0x100000000000001, res: 0x1, name: "and"},
 }
 
 func TestZeroExtension(t *testing.T) {