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

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

Currently, all of the "memri"-style complex operands, which contain
both a register and an immediate, are encoded into a single field in
the instruction definition. This requires complex encoders/decoders,
and instruction definitions that insert and extract the correct parts
of the bits.

Now, switch to naming and encoding/decoding the sub-operands
separately.

Thus, we can now disable useDeprecatedPositionallyEncodedOperands.

Reviewed By: barannikov88

Differential Revision: https://reviews.llvm.org/D137670