[DAGCombine] reduceBuildVecToShuffle(): sort input vectors by decreasing size

The sorting, obviously, must be stable, else we will have random assembly fluctuations.

Apparently there was no test coverage that would benefit from that,
so i've added one test.

The sorting consists of two parts - just sort the input vectors,
and recompute the shuffle mask -> input vector mapping.
I don't believe we need to do anything else.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D104187