[DAGCombiner] Do not fold truncate(build_vector(..)) if it creates an illegal type

Summary:
It can be the case that a vector type is legal but the corresponding
scalar type is not legal for an architecture (i8 vs. v16i8 on AArch64).
Check if the scalar type created when folding
  truncate(build_vector(x,y)) -> build_vector(truncate(x),truncate(y))

is legal if we are running after the type legalizer.

This fixes https://github.com/android/ndk/issues/1207.

Reviewers: RKSimon, srhines

Subscribers: kristof.beyls, hiraditya, danielkiss, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76312