[SLP]Fix legality checks for bswap-based transformations
Fix the checks for the non-power-of-2 base bswaps by checking the
power-of-2 of the source type, not the target scalar type. Plus, add
cost estimation for zext, if the source type does not match the scalar type and fixes final bitcasting for the reduced values.
Fixes https://github.com/llvm/llvm-project/pull/184018#issuecomment-4053477562
