[mips][msa] Added support for matching insert and copy from normal IR (i.e. not intrinsics)

Changes to MIPS SelectionDAG:
* Added nodes VEXTRACT_[SZ]EXT_ELT to represent extract and extend in a single
  operation and implemented the DAG combines necessary to fold sign/zero
  extends into the extract.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191199 91177308-0d34-0410-b5e6-96231b3b80d8