[llvm] [GlobalIsel] Combine trunc of binop (PR #107721)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

For these binops constants are only on the RHS:
https://github.com/llvm/llvm-project/blob/247d3ea843cb20d8d75ec781cd603c8ececf8934/llvm/include/llvm/Target/GlobalISel/Combine.td#L481

G_SUB might be an exception.

https://github.com/llvm/llvm-project/pull/107721