[x86] add load fold patterns for movddup with vzext_load

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350005 91177308-0d34-0410-b5e6-96231b3b80d8