[AArch64][GlobalISel] Lower G_SHUFFLE_VECTOR with 1 elt src and 1 elt mask.

Again, it's weird that these are allowed. Since lowering support was added in
r368709 we started crashing on compiling the neon intrinsics test in the test
suite. This fixes the lowering to fold the 1 elt src/mask case into copies.

llvm-svn: 369135