[x86] add load fold patterns for movddup with vzext_load

The missed load folding noticed in D55898 is visible independent of that change 
either with an adjusted IR pattern to start or with AVX2/AVX512 (where the build 
vector becomes a broadcast first; movddup is not produced until we get into isel 
via tablegen patterns).

Differential Revision: https://reviews.llvm.org/D55936

llvm-svn: 350005