[mips] Fix handling of instructions which copy to/from accumulator registers.

Expand copy instructions between two accumulator registers before callee-saved
scan is done. Handle copies between integer GPR and hi/lo registers in
MipsSEInstrInfo::copyPhysReg. Delete pseudo-copy instructions that are not
needed.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180827 91177308-0d34-0410-b5e6-96231b3b80d8