[RISCV] Add TTI support for cpop with Zbb

This will tell loop idiom recognize that it can make popcount loops countable
using the ctpop intrinsic. I didn't bother checking for illegal types.
Type legalization knows how to split a ctpop into multiple ctops added together.
Assuming we only receive reasonable integer bit widths, a few cpop instructions
added together is probably better than the loop.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D99203