[AArch64] Turn truncating buildvectors into truncates

When lowering large v16f32->v16i8 fp_to_si_sat, the fp_to_si_sat node is
split several times, creating an illegal v4i8 concat that gets expanded
into a BUILD_VECTOR. After some combining and other legalisation, it
ends up the a buildvector that extracts from 4 vectors, looking like
BUILDVECTOR(a0,a1,a2,a3,b0,b1,b2,b3,c0,c1,c2,c3,d0,d1,d2,d3). That is
really an v16i32->v16i8 truncate in disguise.

This adds a ReconstructTruncateFromBuildVector method to detect the
pattern, converting it back into the legal "concat(trunc(concat(trunc(a),
trunc(b))), trunc(concat(trunc(c), trunc(d))))" tree. The extracted
nodes could also be v4i16, in which case the truncates are not needed.
All those truncates and concats then become uzip1's, which is much
better than expanding by moving vector lanes around.

Differential Revision: https://reviews.llvm.org/D119469