[mips] [IAS] Do not generate redundant move when expanding lw/sw with symbol.

Summary:
Even though there is no 2nd register operand in the "lw/sw $8, symbol" case, we still try to find one, 
and we end up with $0, which makes us generate an unnecessary "addu $8, $8, $0" (a.k.a. "move $8, $8").

We can avoid this by checking if the 2nd register operand is different from $0, before generating the addu.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8055

llvm-svn: 234406