[DAGCombine] `combineShuffleToZeroExtendVectorInReg()`: widen shuffle elements...

[DAGCombine] `combineShuffleToZeroExtendVectorInReg()`: widen shuffle elements before trying to match

We might have sunk a bitcast into shuffle, and now it might be operating
on more fine-grained elements than what we'd match, so we must not be
dependent on whatever the granularity the shuffle happened to be in,
but transform it into the one canonical for us - with widest elements.