[GVNHoist] Re-enable GVNHoist by default

Rebase rL341954 since https://bugs.llvm.org/show_bug.cgi?id=38912
has been fixed by rL342055.

Precommit testing performed:
* Overnight runs of csmith comparing the output between programs
  compiled with gvn-hoist enabled/disabled.
* Bootstrap builds of clang with UbSan/ASan configurations.

llvm-svn: 342387