[clang] [llvm] Reimplement constrained 'trunc' using operand bundles (PR #118253)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> llvm.trunc is currently marked IntrNoMem in Intrinsics.td; you'll need to update that if you want it to read/modify FP state. (Trying to override the default by sticking attributes on top doesn't work properly, as far as I know.)

I think we need a dedicated fp env attribute to model this 

https://github.com/llvm/llvm-project/pull/118253