[x64] Use smaller code sequence when pushing SMIs

If exactly one byte in the upper half of the pushed 8-byte value is
set, use a smaller code sequence to push this value on the stack.
Before, we did
  movq r10,<constant>
  push r10
Now, we do
  push 0x0
  movb [rsp+<offset>],<byte>

The old sequence had 12 bytes, the new one has 7.
Pushing such values is used a lot for stack frame markers, which are
small numbers (1-17) encoded as SMIs.

Review-Url: https://codereview.chromium.org/2685213004
Cr-Commit-Position: refs/heads/master@{#43146}

src/x64/macro-assembler-x64.cc

@@ -2465,10 +2465,19 @@ void MacroAssembler::Push(Smi* source) {
   intptr_t smi = reinterpret_cast<intptr_t>(source);
   if (is_int32(smi)) {
     Push(Immediate(static_cast<int32_t>(smi)));
-  } else {
+    return;
+  }
+  int first_byte_set = base::bits::CountTrailingZeros64(smi) / 8;
+  int last_byte_set = (63 - base::bits::CountLeadingZeros64(smi)) / 8;
+  if (first_byte_set == last_byte_set && kPointerSize == kInt64Size) {
+    // This sequence has only 7 bytes, compared to the 12 bytes below.
+    Push(Immediate(0));
+    movb(Operand(rsp, first_byte_set),
+         Immediate(static_cast<int8_t>(smi >> (8 * first_byte_set))));
+    return;
+  }
   Register constant = GetSmiConstant(source);
   Push(constant);
-  }
 }