[RISCV] Silence an unused variable warning in release builds [NFC]

Summary:
Also minor cleanups:
1. Avoided multiple calls to Fixup.getKind()
2. Avoided multiple calls to getFixupKindInfo()
3. Removed a redundant return.

Reviewers: asb, apazos

Reviewed By: asb

Subscribers: rbar, johnrusso, llvm-commits

Differential Revision: https://reviews.llvm.org/D39881

llvm-svn: 317908