[RISCV][test] Rename prefix and remove unused text (NFC)

In D123418 we removed some RUN line (ex. RV32-ELEN16) but their
expected results still exist there.
Remove them and rename prefix for more descriptive.

Reviewed By: frasercrmck, asb, craig.topper

Differential Revision: https://reviews.llvm.org/D124179