[clang] [llvm] [RISCV] Add partial support for -fzero-call-used-regs (PR #194883)

[Sam Elliott via cfe-commits]

https://github.com/lenary commented:

This is coming together well, I think. Most of the comments below are small-scale.

For the tests, it would be good to have some unused arguments, and also maybe try to make the tests use some non-argument gprs, so you can show when you're just zeroing arg registers.

https://github.com/llvm/llvm-project/pull/194883