[NFCI][MemoryTaggingSupport] Replace getAllocatedType with getAllocationSize (#177436)

Replace the `isSized()` and `isScalableTy()` type predicate checks with
a more direct `getAllocationSize()` query. The `isSized` check seems
unnecessary since the verifier already rejects unsized alloca.

For context, I'm working on deleting (or at least minimizing uses of)
the getAllocatedType function, since many optimizations need to destroy
that info and just produce byte sized allocas. It is for the same
reason that opaque pointers were introduced–memory doesn't inherently
have a provenience type, only uses (load and stores) do.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
GitOrigin-RevId: 5188d2e7454f101109fa0c6c80322148927f5f35