[SelectionDAG] Remove invalid TypeSize conversion from PromoteIntRes_BITCAST.

Extend the TypeWidenVector case of PromoteIntRes_BITCAST to work
with TypeSize directly rather than silently casting to unsigned.

To accomplish this I've extended TypeSize with an interface that
essentially allows TypeSize division when both operands have the
same number of dimensions.

There still exists combinations of scalable vector bitcasts that
cause compiler crashes. I call these out by adding "is missing"
entries to sve-bitcast.

Depends on D126957.
Fixes: #55114

Differential Revision: https://reviews.llvm.org/D127126