[PowerPC] Switch to by-name matching for instructions (part 1 of 2).

This is a follow-on to https://reviews.llvm.org/D134073.

After https://reviews.llvm.org/D137653 we can now switch the PPC
target away from positional operand matching.

This patch fixes all of the "easy" cases. While this changes a large
number of lines of tablegen source, it results in only a single
non-comment change in the code generated by tablegen: the (unused)
codegen-only "MTVRSAVEv" instruction was previously incorrectly
encoding operand 0, and now encodes (correctly) operand 1.

Changes which result in generated-code changes have been split off
into the next (smaller) patch, for ease of review.

Reviewed By: barannikov88

Differential Revision: https://reviews.llvm.org/D137661