[ms-inline asm] Add support for numeric displacement expressions in bracketed
memory operands.

Essentially, this layers an infix calculator on top of the parsing state
machine.  The scale on the index register is still expected to be an immediate

 __asm mov eax, [eax + ebx*4]

and will not work with more complex expressions.  For example,

 __asm mov eax, [eax + ebx*(2*2)]

The plus and minus binary operators assume the numeric value of a register is
zero so as to not change the displacement.  Register operands should never
be an operand for a multiply or divide operation; the scale*indexreg
expression is always replaced with a zero on the operand stack to prevent
such a case.
rdar://13521380