[x86] be more selective about converting 'and' to shuffle (PR37749)

isVectorClearMaskLegal() is the TLI hook used by the generic
DAGCombiner::XformToShuffleWithZero().

We've grown to accomodate/expect this transform to shuffle
(disabling it more generally results in many regressions).
So I'm narrowly excluding the 256-bit types that clearly 
are not worthwhile for AVX1. 

I think in most cases we are able to recover by converting 
the shuffle back into 'and' ops, but the cases in:
https://bugs.llvm.org/show_bug.cgi?id=37749
...show that there are cracks.

llvm-svn: 334759