[GVNHoist] Re-enable GVNHoist by default

My initial motivation for this came from https://reviews.llvm.org/D48122,
where it was pointed out that my change didn't fit well in SimplifyCFG and
therefore using GVNHoist was a better way to go. GVNHoist has been disabled
for a while as there was a list of bugs related to it.

I have fixed the following bugs:

https://bugs.llvm.org/show_bug.cgi?id=37808 -> https://reviews.llvm.org/D48372 (rL337149)
https://bugs.llvm.org/show_bug.cgi?id=36787 -> https://reviews.llvm.org/D49555 (rL337674)
https://bugs.llvm.org/show_bug.cgi?id=37445 -> https://reviews.llvm.org/D49425 (rL337680)

The next two bugs no longer occur, and it's unclear which commit fixed them:

https://bugs.llvm.org/show_bug.cgi?id=36635
https://bugs.llvm.org/show_bug.cgi?id=37791

I investigated this one and proved to be unrelated to GVNHoist, but a genuine bug in NewGvn:

https://bugs.llvm.org/show_bug.cgi?id=37660

To convince myself GVNHoist is in a good state I made a successful bootstrap build of LLVM.
Merging this change now in order to make it to the LLVM 7.0.0 branch.

Differential Revision: https://reviews.llvm.org/D49858

llvm-svn: 338240