[mips] [IAS] Add support for LASym with identical source and destination register operands.

Summary:
In this case, we're supposed to load the address of the symbol in AT and then ADDu it with the source register and
put it in the destination register.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9366

llvm-svn: 240273