[GISel][AArch64] Combine G_BUILD_VECTOR(G_UNMERGE) with undef elements

This extends the existing legalization combine to fold G_BUILD_VECTOR where the
sources are all from the same G_UNMERGE, to handle cases where some of the
lanes are undef. This comes up in the legalization of <3 x ..> vectors in
AArch64, where they are padded with undef.

There are two choices for what to create. This patch just removes the
G_BUILD_VECTOR/G_UNMERGE, losing the information about which lanes are undef.
The alternative would be to generate an identity G_SHUFFLE_VECTOR with undef
lanes marked as undef. I think both have advantages and disadvantages.

Differential Revision: https://reviews.llvm.org/D158063