[SVE] Fix incorrect code generation for bitcasts of unpacked vector types.

Bitcasting between unpacked scalable vector types of different
element counts is not a NOP because the live elements are laid out
differently.
               01234567
e.g. nxv2i32 = XX??XX??
     nxv4f16 = X?X?X?X?

Differential Revision: https://reviews.llvm.org/D126957