cmd/compile: mark BS{F,R}L as clobbering flags

Turns out it doesn't currently matter, as these ops are always issued
together with a BTSL which does clobber flags. So I can't write a test
that currently fails. But better to be future-proof.

BS{F,R}Q generates flags, so it doesn't need to be marked as clobbering.

Change-Id: I70daea154023fd435fac696bf3a384803c647cd3
Reviewed-on: https://go-review.googlesource.com/117375
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/ssa/gen/AMD64Ops.go

@@ -350,9 +350,9 @@ func init() {
 		// flags are set to "equal" if the input is zero, "not equal" otherwise.
 		// BS{F,R}L returns only the result.
 		{name: "BSFQ", argLength: 1, reg: gp11flags, asm: "BSFQ", typ: "(UInt64,Flags)"},        // # of low-order zeroes in 64-bit arg
-		{name: "BSFL", argLength: 1, reg: gp11, asm: "BSFL", typ: "UInt32"},              // # of low-order zeroes in 32-bit arg
+		{name: "BSFL", argLength: 1, reg: gp11, asm: "BSFL", typ: "UInt32", clobberFlags: true}, // # of low-order zeroes in 32-bit arg
 		{name: "BSRQ", argLength: 1, reg: gp11flags, asm: "BSRQ", typ: "(UInt64,Flags)"},        // # of high-order zeroes in 64-bit arg
-		{name: "BSRL", argLength: 1, reg: gp11, asm: "BSRL", typ: "UInt32"},              // # of high-order zeroes in 32-bit arg
+		{name: "BSRL", argLength: 1, reg: gp11, asm: "BSRL", typ: "UInt32", clobberFlags: true}, // # of high-order zeroes in 32-bit arg
 
 		// CMOV instructions: 64, 32 and 16-bit sizes.
 		// if arg2 encodes a true result, return arg1, else arg0

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

@@ -7395,6 +7395,7 @@ var opcodeTable = [...]opInfo{
 	{
 		name:         "BSFL",
 		argLen:       1,
+		clobberFlags: true,
 		asm:          x86.ABSFL,
 		reg: regInfo{
 			inputs: []inputInfo{
@@ -7422,6 +7423,7 @@ var opcodeTable = [...]opInfo{
 	{
 		name:         "BSRL",
 		argLen:       1,
+		clobberFlags: true,
 		asm:          x86.ABSRL,
 		reg: regInfo{
 			inputs: []inputInfo{