Re-adds tests that were accidentally removed.
Implements %mem8, %mem16, %mem32, and %mem64 forms.

The %mem8, %mem16, %mem32, and %mem64 forms specify 8-bit, 16-bit,
32-bit, and 64-bit memory operands (respectively).  The %mem
form (still) requires the compiler and assembler to infer the operand
size.