`MCInstrDesc` provides the `mayLoad` and `mayStore` flags that seem
appropriate to use as a target-independent way to implement `isLoad` and
`isStore`.
I believe this is currently good enough to use for the RISC-V target as
well. I've provided a test for this that checks the generated dyno
stats (which seems to be the only thing both `isLoad` and `isStore` are
used for).
Reviewed By: maksfb
Differential Revision: https://reviews.llvm.org/D159266
76f040bda6