[RISCV] Add vector integer mul/mulh/div/rem ISel patterns

There is no test coverage for the mulhs or mulhu patterns as I can't get
the DAGCombiner to generate them for scalable vectors. There are a few
places in that still need updating for that to work. I left the patterns
in regardless as they are correct.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D94073