intel/compiler: Very small shift optimizations

Open Ian Romanick requested to merge idr/mesa:review/shift-right-right into main

The two "meaty" commits here, intel/compiler: Relax some conditions in try_copy_propagate and intel/compiler: Force count parameter of shift to be signed, came out of some work I was doing for !7698 (merged). I decided to split them into a separate MR rather than mash them into that MR.

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