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

[Benji Smith via llvm-commits]

Benjins wrote:

> I wonder whether LLVMGetCanUseFastMathFlags should be just LLVMCanUseFastMathFlags.

Sure. I pushed a commit that renames it to `LLVMCanValueUseFastMathFlags`, which should make it more clear that it's for individual values

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