[RISCV][test] Add additional check-prefixes to fix conflicts in tests (#160689)
These are all cases where check lines were being silently dropped prior
to #159321 which added proper warnings.
I did `find llvm/test/CodeGen/RISCV -name "*.ll" -exec
./llvm/utils/update_llc_test_checks.py --llc-bin=./remote-llc -u {} \;`
and went through all cases that emitted the new warning.
`idiv_large.ll` is a case that seems to not be generated by
update_llc_test_checks but still has the comment indicating it was
(presumably it was hand-edited after generation).