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

[Derek Schuff via llvm-commits]

dschuff wrote:

So my first thought was that this should be emitting some sort of bitmask to zero out the top 16 bits, but I can see that the DAG isel doesn't do that either. Do you know what the calling convention is in this case? I assume you've checked that the final behavior is correct?
@sunfishcode do you recall whether fast-isel handles these odd-sized bitwidth, or those get expanded out somewhere before?

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