cmd/compile/internal/amd64: update popcnt code generation

Popcnt has false dependency on output register and generates
MOVQ $0, reg to break it. But recently we switched MOVQ $0, reg
encoding from xor reg, reg  to actual mov $0, reg. This CL updates
code generation for popcnt to use actual XOR.

Change-Id: I4c1fc11e85758b53ba2679165fa55614ec54b27d
Reviewed-on: https://go-review.googlesource.com/82516
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/amd64/ssa.go

@@ -867,10 +867,10 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 	case ssa.OpAMD64POPCNTQ, ssa.OpAMD64POPCNTL:
 		if v.Args[0].Reg() != v.Reg() {
 			// POPCNT on Intel has a false dependency on the destination register.
-			// Zero the destination to break the dependency.
-			p := s.Prog(x86.AMOVQ)
-			p.From.Type = obj.TYPE_CONST
-			p.From.Offset = 0
+			// Xor register with itself to break the dependency.
+			p := s.Prog(x86.AXORQ)
+			p.From.Type = obj.TYPE_REG
+			p.From.Reg = v.Reg()
 			p.To.Type = obj.TYPE_REG
 			p.To.Reg = v.Reg()
 		}