[mips] LLVM and GAS now use same instructions for CFA Definition. NFCI

LLVM previously used `DW_CFA_def_cfa` instruction in .eh_frame to set
the register and offset for current CFA rule. We change it to
`DW_CFA_def_cfa_register` which is the same one used by GAS that only
changes the register but keeping the old offset.

Patch by Mirko Brkusanin.

Differential Revision: https://reviews.llvm.org/D61899

llvm-svn: 360765