Add flag to PassManagerBuilder to disable GVN Hoist Pass.

Summary:
Adding a flag to diable GVN Hoisting by default.
Note: The GVN Hoist Pass causes some Halide tests to hang. Halide will disable the pass while investigating.

Reviewers: llvm-commits, chandlerc, spop, dberlin

Subscribers: mehdi_amini

Differential Revision: https://reviews.llvm.org/D22639

llvm-svn: 276479