Enabling displacing existing MemoryOperands inside of inline assembly.

[pgoodman]

The following should be possible:

MemoryOperand foo(...);
lir::InlineAssembly asm_(foo);
asm_.InlineBefore(instr,
    "INC m64 [%0 + 4];"_x86_64);

Normally, one would only be able to do "INC m64 %0", so I think this would make a nice improvement that would reduce the number of parameters that need to be passed in to inline assembly blocks.