[X86] Declare SSE4.1/AVX2 vector extloads covered by PMOV[SZ]X legal.

Now that we can fully specify extload legality, we can declare them
legal for the PMOVSX/PMOVZX instructions.  This for instance enables
a DAGCombine to fire on code such as
  (and (<zextload-equivalent> ...), <redundant mask>)
to turn it into:
  (zextload ...)
as seen in the testcase changes.

There is one regression, in widen_load-2.ll: we're no longer able
to do store-to-load forwarding with illegal extload memory types.
This will be addressed separately.

Differential Revision: http://reviews.llvm.org/D6533

llvm-svn: 226676