[RISCV] Indicate that RISCVMergeBaseOffsetOpt preserves the CFG.

Return false from runOnFunction if nothing changed. Curiously
we already returned a bool from detectAndFoldOffset, but didn't
use it.

Fix a couple breaks after returns that I saw while auditing
detectAndFoldOffset.

Differential Revision: https://reviews.llvm.org/D107303