[DAGCombiner] Teach combineShiftToMULH to handle constant and const splat vector.

Fold (srl (mul (zext i32:$a to i64), i64:c), 32) -> (mulhu $a, $b),
if c can truncate to i32 without loss.

Reviewed By: frasercrmck, craig.topper, RKSimon

Differential Revision: https://reviews.llvm.org/D108129