[SVE] Disable some BUILD_VECTOR related code generator features.

Fixed length vector code generation for SVE does not yet custom
lower BUILD_VECTOR and instead relies on expansion.  At the same
time custom lowering for VECTOR_SHUFFLE is also not available so
this patch updates isShuffleMaskLegal to reject vector types that
require SVE.

Related to this it also prevents the merging of stores after
legalisation because this only works when BUILD_VECTOR is either
legal or can be elminated.  When this is not the case the code
generator enters an infinite legalisation loop.

Differential Revision: https://reviews.llvm.org/D83408