[Sparc] Fix useDeprecatedPositionallyEncodedOperands errors.

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

It renames a few fields to have consistent names, as well as renaming
operands to match the field names.

Behavior is unchanged by this cleanup. (The only generated code change
is for the disassembler for LDSTUB/LDSTUBA, but in both old and new
versions, it fails to add enough operands, and thus triggers a runtime
abort. I will address that bug in a future commit.)

Differential Revision: https://reviews.llvm.org/D134201