[DAGCombiner] reduce buildvec of zexted extracted element to shuffle

The motivating case for this is shown in the first regression test. We are 
transferring to scalar and back rather than just zero-extending with 'vpmovzxdq'.

That's a special-case for a more general pattern as shown here. In all tests, 
we're avoiding the vector-scalar-vector moves in favor of vector ops.

We aren't producing optimal shuffle code in some cases though, so the patch is
limited to reduce regressions.

Differential Revision: https://reviews.llvm.org/D56281

llvm-svn: 351198