[llvm] [C API] Add getters and setters for fast-math flags on relevant instructions (PR #75123)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM, though I wonder whether `LLVMGetCanUseFastMathFlags` should be just `LLVMCanUseFastMathFlags`.

We don't seem to have anything using "LLVMCan" right now, but we do have "LLVMHas" and "LLVMIs".

https://github.com/llvm/llvm-project/pull/75123