Revert "cmd/compile: optimize 386's math.bits.TrailingZeros16"

This reverts CL 189277.

Reason for revert: broke 32-bit builders.

Updates golang#33902

Change-Id: Ie5f180d0371a90e5057ed578c334372e5fc3a286
Reviewed-on: https://go-review.googlesource.com/c/go/+/192097
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

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

@@ -3370,7 +3370,7 @@ func init() {
 		func(s *state, n *Node, args []*ssa.Value) *ssa.Value {
 			return s.newValue1(ssa.OpCtz16, types.Types[TINT], args[0])
 		},
-		sys.AMD64, sys.ARM, sys.I386, sys.ARM64, sys.Wasm)
+		sys.AMD64, sys.ARM, sys.ARM64, sys.Wasm)
 	addF("math/bits", "TrailingZeros16",
 		func(s *state, n *Node, args []*ssa.Value) *ssa.Value {
 			x := s.newValue1(ssa.OpZeroExt16to64, types.Types[TUINT64], args[0])

src/cmd/compile/internal/ssa/gen/386.rules

@@ -66,8 +66,6 @@
 
 (Sqrt x) -> (SQRTSD x)
 
-(Ctz16 x) -> (BSFL (ORLconst <typ.UInt32> [0x10000] x))
-
 // Lowering extension
 (SignExt8to16  x) -> (MOVBLSX x)
 (SignExt8to32  x) -> (MOVBLSX x)

test/codegen/mathbits.go

@@ -296,7 +296,6 @@ func TrailingZeros32(n uint32) int {
 
 func TrailingZeros16(n uint16) int {
 	// amd64:"BSFL","BTSL\\t\\$16"
-	// 386:"BSFL\t"
 	// arm:"ORR\t\\$65536","CLZ",-"MOVHU\tR"
 	// arm64:"ORR\t\\$65536","RBITW","CLZW",-"MOVHU\tR",-"RBIT\t",-"CLZ\t"
 	// s390x:"FLOGR","OR\t\\$65536"