Allow x86 memory operands for 64-bit popcnt on x86.

By Mark's request.
This allows:

 popcnt edx, [esp + 16]
 popcnt ebx, [esp + 20]
 add ebx, edx
 .....

instead of the more elaborate

 mov edx, [ecx + 16]
 mov ebx, [ecx + 20]
 popcnt eax, edx
 popcnt ebp, ebx
 add ebp, eax

Change-Id: Iea30ad7b9a1aba8591e3dbb72e24ef12e81cc2ce