[AArch64] Custom lower concat(v4i8 load, ...)

We already have custom lowering for v4i8 load, which loads as a f32,
converts to a vector and bitcasts and extends the result to a v4i16.
This adds some custom lowering of concat(v4i8 load, ...) to keep the
result as an f32 and create a buildvector of the resulting f32 loads.
This helps not create all the extends and bitcasts, which are often
difficult to fully clean up.

Differential Revision: https://reviews.llvm.org/D121400