[Liftoff] Use AVX instructions if available

Mixing SSE and AVX instructions can cause severe slowdowns, so always
use AVX instructions if available.
The {Movss} method of {TurboAssembler} automatically does this,
emitting either {vmovss} or {movss}.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I45124944964e5a970814c7b50dfbfca333137dc1
Reviewed-on: https://chromium-review.googlesource.com/865910Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50572}

src/wasm/baseline/x64/liftoff-assembler-x64.h

@@ -194,7 +194,7 @@ void LiftoffAssembler::Move(LiftoffRegister dst, LiftoffRegister src) {
   if (dst.is_gp()) {
     movq(dst.gp(), src.gp());
   } else {
-    movsd(dst.fp(), src.fp());
+    Movsd(dst.fp(), src.fp());
   }
 }
 
@@ -453,7 +453,7 @@ void LiftoffAssembler::PushCallerFrameSlot(const VarState& src,
         pushq(src.reg().gp());
       } else {
         subp(rsp, Immediate(kStackSlotSize));
-        movsd(Operand(rsp, 0), src.reg().fp());
+        Movsd(Operand(rsp, 0), src.reg().fp());
       }
       break;
     case VarState::kI32Const:
@@ -476,7 +476,7 @@ void LiftoffAssembler::PushRegisters(LiftoffRegList regs) {
     unsigned offset = 0;
     while (!fp_regs.is_empty()) {
       LiftoffRegister reg = fp_regs.GetFirstRegSet();
-      movsd(Operand(rsp, offset), reg.fp());
+      Movsd(Operand(rsp, offset), reg.fp());
       fp_regs.clear(reg);
       offset += sizeof(double);
     }
@@ -489,7 +489,7 @@ void LiftoffAssembler::PopRegisters(LiftoffRegList regs) {
   unsigned fp_offset = 0;
   while (!fp_regs.is_empty()) {
     LiftoffRegister reg = fp_regs.GetFirstRegSet();
-    movsd(reg.fp(), Operand(rsp, fp_offset));
+    Movsd(reg.fp(), Operand(rsp, fp_offset));
     fp_regs.clear(reg);
     fp_offset += sizeof(double);
   }