[CodeGen] Don't combine extract + concat vectors with non-legal types

[CodeGen] Don't combine extract + concat vectors with non-legal types

Summary:
The following combine currently breaks in the DAGCombiner:

```
extract_vector_elt (concat_vectors v4i16:a, v4i16:b), x
   -> extract_vector_elt a, x
```

This happens because after we have combined these nodes we have inserted nodes
that use individual instances of the vector element type. In the above example
i16. However this isn't a legal type on all backends, and when the combining pass calls
the legalizer it breaks as it expects types to already be legal. The type legalizer has
already been run, and running it again would make a mess of the nodes.

In the example code at least, the generated code is still efficient after the change.

Reviewers: miyuki, arsenm, dmgreen, lebedev.ri

Reviewed By: miyuki, lebedev.ri

Subscribers: lebedev.ri, wdng, hiraditya, steven.zhang, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83231