[assembler][x64] Revert to longer move immediate instruction

The shorter xorl + movb instructions cause regressions.

Bug: chromium:1220093
Change-Id: Ib7da0ea16669fa490b78a8095f6f3b35e972e1cb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2964396Reviewed-by: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75161}

src/codegen/x64/macro-assembler-x64.h

@@ -251,9 +251,10 @@ class V8_EXPORT_PRIVATE TurboAssembler : public SharedTurboAssembler {
   void Move(Register dst, intptr_t x) {
     if (x == 0) {
       xorl(dst, dst);
-    } else if (is_uint8(x)) {
-      xorl(dst, dst);
-      movb(dst, Immediate(static_cast<uint32_t>(x)));
+      // The following shorter sequence for uint8 causes performance
+      // regressions:
+      // xorl(dst, dst); movb(dst,
+      // Immediate(static_cast<uint32_t>(x)));
     } else if (is_uint32(x)) {
       movl(dst, Immediate(static_cast<uint32_t>(x)));
     } else if (is_int32(x)) {