[SelectionDAGBuilder] Update signature of `getRegsAndSizes()`.

The mapping between registers and relative size has been updated to
use TypeSize to account for the size of scalable EVTs.

The patch is a NFCI, if not for the fact that with this change the
function `getUnderlyingArgRegs` does not raise a warning for implicit
conversion of `TypeSize` to `unsigned` when generating machine code
from the test added to the patch.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D92096