[GlobalISel] Detect splats built with G_CONCAT_VECTORS

Add support to the MI matching of vector splats for patterns that
consist of `G_CONCAT_VECTORS` of smaller splats with the same constant
value. With this, we would consider the following pseudo-MIR to be a splat:

```
%0 = G_[F]CONSTANT [...]
%1 = G_BUILD_VECTOR %0, %0, ..., %0
%2 = G_CONCAT_VECTORS %1, %1, ..., %1
```

Since it uses recursion for matching splats, it could match pretty
complicated patterns with all sorts of combinations of `G_BUILD_VECTOR`
and `G_CONCAT_VECTORS` (e.g. a `G_CONCAT_VECTORS` with
a `G_BUILD_VECTOR_TRUNC` and another `G_CONCAT_VECTORS` as operands),
and it should also look through copies etc.

This should make it easier to match complex immediates for certain
instructions on AMDGPU, where for instance a <8 x s16> will be split
before instruction selection into a `G_CONCAT_VECTORS` of <2 x s16>
splats.

Differential Revision: https://reviews.llvm.org/D141902