[llvm] [IR] Store fast-math flags in subclasses of Instruction (PR #191190)

[Joshua Cranmer via llvm-commits]

jcranmer-intel wrote:

What's the compile-time impact of this change? I did some experiments for https://discourse.llvm.org/t/rfc-changing-llvm-value-layout/79257, and found that I could scrounge more bits from `Value` with sub-1% compile-time hit.

https://github.com/llvm/llvm-project/pull/191190