[GISel][CombinerHelper] Add support for scalar type for the result of shuffle vector

LLVM IR of 1-element vectors get lower into scalar in GISel. As a
result, shuffle vector may also produce a scalar.

This patch teaches the shuffle combiner how to deal with scalars when
they are in the destination type of a shuffle vector.

For now, we just support the easy case where this can be lowered to
a plain copy. For other cases, we leave the shuffle vector as is.

This type of IR are seen in O0 pipelines. E.g., as produced with
SingleSource/UnitTests/Vector/AArch64/aarch64_neon_intrinsics.c.

rdar://problem/57198904