[Power9] Part-word VSX integer scalar loads/stores and sign extend instructions

This patch corresponds to review:
https://reviews.llvm.org/D23155

This patch removes the VSHRC register class (based on D20310) and adds
exploitation of the Power9 sub-word integer loads into VSX registers as well
as vector sign extensions.
The new instructions are useful for a few purposes:

    Int to Fp conversions of 1 or 2-byte values loaded from memory
    Building vectors of 1 or 2-byte integers with values loaded from memory
    Storing individual 1 or 2-byte elements from integer vectors

This patch implements all of those uses.

llvm-svn: 283190