[mips] [IAS] Give expandLoadAddressSym() more specific arguments. NFC.

Summary:
If we only pass the necessary operands, we don't have to determine the position of the symbol operand when entering expandLoadAddressSym().
This simplifies the expandLoadAddressSym() code.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9291

llvm-svn: 237355