[mips][msa] Pattern match the splat.d instruction

Introduced a new pattern for matching splat.d explicitly.

Both splat.d and splati.d can now be generated from the @llvm.mips.splat.d
intrinsic depending on whether an immediate value has been passed.

Differential Revision: https://reviews.llvm.org/D45683

llvm-svn: 331771