[turbofan] Turn Math.clz32 into an inlinable builtin.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index 98a76ad58e6f2c4440684e0679b3292e910f4437..d1683e6ae7ab9685d64c9801a78891f2f56a2cb1 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -4001,14 +4001,8 @@ void LCodeGen::DoMathLog(LMathLog* instr) {
 void LCodeGen::DoMathClz32(LMathClz32* instr) {
   Register input = ToRegister(instr->value());
   Register result = ToRegister(instr->result());
-  Label not_zero_input;
-  __ bsrl(result, input);
 
-  __ j(not_zero, &not_zero_input);
-  __ Set(result, 63);  // 63^31 == 32
-
-  __ bind(&not_zero_input);
-  __ xorl(result, Immediate(31));  // for x in [0..31], 31^x == 31-x.
+  __ Lzcntl(result, input);
 }