InstX8632Store is essentially a "mov" and it would emit
a mov, but it did not add the ss/sd suffix based on the operand type.

Also, there are some cases where legalization would leave
two memory operands in the case that one of them
is a floating point immediate:

storeDoubleConst:
.LstoreDoubleConst$entry:
  mov     eax, dword ptr [esp+4]
  mov     qword ptr [eax], qword ptr [L$double$1]
  ret

BUG=none
R=stichnot@chromium.org, wala@chromium.org

Review URL: https://codereview.chromium.org/341683002