[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