Refine a bit ClampXToUint8

src/ia32/macro-assembler-ia32.cc

Index: src/ia32/macro-assembler-ia32.cc
diff --git a/src/ia32/macro-assembler-ia32.cc b/src/ia32/macro-assembler-ia32.cc
index 0235a13894bd08bcf84d9d9f8f06cf4009ed26a2..1e7320545390f64fe27c4c7d8ea2af4097d3ae91 100644
--- a/src/ia32/macro-assembler-ia32.cc
+++ b/src/ia32/macro-assembler-ia32.cc
@@ -220,16 +220,14 @@ void MacroAssembler::ClampDoubleToUint8(XMMRegister input_reg,
   j(zero, &done, Label::kNear);
   cmp(result_reg, Immediate(0x1));
   j(overflow, &conv_failure, Label::kNear);
-  mov(result_reg, Immediate(0));
-  setcc(sign, result_reg);
-  sub(result_reg, Immediate(1));
-  and_(result_reg, Immediate(255));
+  sar(result_reg, 31);
+  not_(result_reg);  // No zero extension seems to be safe here.
   jmp(&done, Label::kNear);
   bind(&conv_failure);
-  Move(result_reg, Immediate(0));
+  sar(result_reg, 31);
   ucomisd(input_reg, scratch_reg);
-  j(below, &done, Label::kNear);
-  Move(result_reg, Immediate(255));
+  j(above_equal, &done, Label::kNear);
+  Set(result_reg, Immediate(0));
   bind(&done);
 }
 
@@ -238,8 +236,8 @@ void MacroAssembler::ClampUint8(Register reg) {
   Label done;
   test(reg, Immediate(0xFFFFFF00));
   j(zero, &done, Label::kNear);
-  setcc(negative, reg);  // 1 if negative, 0 if positive.
-  dec_b(reg);  // 0 if negative, 255 if positive.
+  sar(reg, 31);
+  not_(reg);  // No zero extension seems to be safe here.
   bind(&done);
 }