Add missed PostDominatorTree analysis dependency to GVN hoist pass.

Summary:
GVN hoist pass is using PostDominatorTree analysis, therefore the analysis
should be listed in the pass initialization as a dependency.

Reviewed By: sebpop

Differential Revision: https://reviews.llvm.org/D43007

Author: ashlykov <arkady.shlykov@intel.com>
llvm-svn: 324597