[GlobalDCE] Use DenseMap instead of unordered_multimap for GVDependencies.

Summary:
std::unordered_multimap happens to be very slow when the number of elements
grows large. On one of our internal applications we observed a 17x compile time
improvement from changing it to DenseMap.

Reviewers: mehdi_amini, serge-sans-paille, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38916

llvm-svn: 316045