[mips] Use correct frame register for DWARF info when dynamically realigning the stack.

Summary:
This patch overrides TargetFrameLowering::getFrameIndexReference() in order to
specify the correct register when the function needs dynamic stack realignment.
The values returned from this function are used in order to create DW_AT_locations
for DWARF info. These locations would use the wrong registers as it's been
reported in PR25028.

Reviewers: dsanders

Subscribers: dean, llvm-commits

Differential Revision: http://reviews.llvm.org/D13511

llvm-svn: 252882