[AArch64][GlobalISel] Port buildvector -> dup pattern from AArch64ISelLowering

Given this:

```
%x:_(<n x sK>) = G_BUILD_VECTOR %lane, ...
...
%y:_(<n x sK>) = G_SHUFFLE_VECTOR %x(<n x sK>), %foo, shufflemask(0, 0, ...)
```

We can produce:

```
%y:_(<n x sK) = G_DUP %lane(sK)
```

Doesn't seem to be too common, but AArch64ISelLowering attempts to do this
before trying to produce a DUPLANE. Might as well port it.

Also make it so that when the splat has an undef mask, we try setting it to
0. SDAG does this, and it makes sure that when we get the build vector operand,
we actually get a source operand.

Differential Revision: https://reviews.llvm.org/D81979