Expand masked mem intrinsics correctly wrt big-endian

Need to take endianness into account when doing vector to scalar casts
such as %bc = bitcast <8 x i1> %v to i8
Companion commit for https://reviews.llvm.org/D94867
Upload in response to
https://lists.llvm.org/pipermail/llvm-dev/2021-January/147862.html
Attempting to document the actual memory layout rules for vectors in
https://reviews.llvm.org/D94964

Differential Revision: https://reviews.llvm.org/D94765