[clang] [llvm] [RISCV] CodeGen of RVE and ilp32e/lp64e ABIs (PR #76777)

[Alex Bradbury via cfe-commits]

asb wrote:

The RISCVUsage change looks good to me, and it's fantastic that @nemanjai might be able to help push this forward further.

I think all the review comments were addressed, so I'd be happy to merge if @topperc is happy his comments were resolved. Though also fine to wait longer if @nemanjai you wanted to review this as well.

https://github.com/llvm/llvm-project/pull/76777