[llvm] [WebAssembly] Fix trunc in FastISel (PR #138479)

[Derek Schuff via llvm-commits]

dschuff wrote:

I think this change is actually fine, with that explanation. Mostly (aside from the obvious question of "are we sure it's correct" which you answered), I just wanted to make sure that this would be useful; i.e. we wouldn't just immediately run into some other case that caused a bailout for the block, or if we would fall into some indefinite cascade of other things we'd need to add that might be beyond the scope we want. If this change fixes the problem (and it's not just equivalent to directly bailing out), then I'd rather do it than bail out. In general I suspect our fast isel could stand to be maybe a little more complete. I haven't actually looked at stats about how often it bails out recently, but as @sunfishcode said, nobody has looked at it in a long time and it hasn't really been updated with new proposals. So all that to say, I'd rather have this simple addition than a bailout, all else being equal.

https://github.com/llvm/llvm-project/pull/138479