[GlobalISel][AArch64] Gardening: Factor out vector inserts

Factor out the vector insert code in `selectBuildVector`. Replace part of it
with `emitScalarToVector`, since it was pretty much equivalent.

This will make implementing G_INSERT_VECTOR_ELT easier.

Differential Revision: https://reviews.llvm.org/D59322

llvm-svn: 356106