GVN-hoist: enable by default
GVN-hoist: enable by default

As we addressed all compilation time problems with GVN-hoist
https://llvm.org/bugs/show_bug.cgi?id=28670
this patch turns GVN-hoist back by default.

Differential Revision: https://reviews.llvm.org/D23136

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@277685 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm