[llvm] [VPlan] Support `fast` in FMF in VPRecipeWithIRFlags. (PR #130880)

[Luke Lau via llvm-commits]

lukel97 wrote:

I think this is a bit strange because we end up adding a state that doesn't exist in FastMathFlags.

I see that FastMathFlagsTy was used instead of FastMathFlags directly to keep the union tightly packed according to https://reviews.llvm.org/D149079#inline-1447781, i.e. 1 byte instead of the 4 bytes for FastMathFlags

But after fd6619577790f, that union has an unsigned int so it's going to be 4 bytes anyway. So maybe we should just replace FastMathFlagsTy with FastMathFlags directly?

https://github.com/llvm/llvm-project/pull/130880