[mips] Fix expansion of memory operation if destination register is not a GPR.

Summary:
The assembler tries to reuse the destination register for memory operations whenever
it can but it's not possible to do so if the destination register is not a GPR.

Example:
  ldc1 $f0, sym
should expand to:
  lui $at, %hi(sym)
  ldc1 $f0, %lo(sym)($at)

It's entirely wrong to expand to:
  lui $f0, %hi(sym)
  ldc1 $f0, %lo(sym)($f0)

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D4173