[mips] MSA intrinsics header file

This patch adds the msa.h header file containing the shorter names for the
MSA instrinsics, e.g. msa_sll_b for builtin_msa_sll_b.

Reviewers: vkalintiris, zoran.jovanovic

Differential Review: https://reviews.llvm.org/D24674

llvm-svn: 281975