[SPARC][IAS] Rework ASI/Prefetch tag matching in prep for `ParseForAllFeatures`
Unify parts of ASI and Prefetch tag matching at `parseASITag`
and `parsePrefetchTag` to use a common function to parse any immediate
expressions. This introduces a slight regression to error messages,
but is needed so we can enable `ParseForAllFeatures`
in `MatchOperandParserImpl` in a future patch.
Reviewers: jrtc27, brad0, rorth, s-barannikov
Reviewed By: s-barannikov
Pull Request: https://github.com/llvm/llvm-project/pull/96020