[x64] Reduce sub instruction in ComputeCodeStartAddress()

Merge sub with lea if pc_offset is not zero when computing code start address.

Change-Id: I20df6bc1b9a6c1d1f2c97b3de3e85252cea7caec
Reviewed-on: https://chromium-review.googlesource.com/1088318Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Shiyu Zhang <shiyu.zhang@intel.com>
Cr-Commit-Position: refs/heads/master@{#53541}

src/x64/macro-assembler-x64.cc

@@ -2749,10 +2749,7 @@ void TurboAssembler::ComputeCodeStartAddress(Register dst) {
   bind(&current);
   int pc = pc_offset();
   // Load effective address to get the address of the current instruction.
-  leaq(dst, Operand(&current));
-  if (pc != 0) {
-    subq(dst, Immediate(pc));
-  }
+  leaq(dst, Operand(&current, -pc));
 }
 
 void TurboAssembler::ResetSpeculationPoisonRegister() {