[llvm] [RISCV][GISel] Add FP calling convention support (PR #69138)

[Brandon Wu via llvm-commits]

4vtomat wrote:

Do we also want to add test cases for using `float` as incoming/outgoing type? since it uses different register classes other than `$x1-$x31`.

https://github.com/llvm/llvm-project/pull/69138