[GlobalISel] Fold or of shifts to funnel shift.

This change folds a basic funnel shift idiom:
- (or (shl x, amt), (lshr y, sub(bw, amt))) -> fshl(x, y, amt)
- (or (shl x, sub(bw, amt)), (lshr y, amt)) -> fshr(x, y, amt)

This also helps in folding to rotate shift if x and y are equal since we
already have a funnel shift to rotate combine.

Differential Revision: https://reviews.llvm.org/D114499