GVN-hoist: enable by default

[sebpop]

Summary

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.

[sebpop]

Remove "experimental" from pass description.

[probinson]

Would you mind putting test/Feature/optnone-opt.ll back as it was before r276479? That would put the pass back in the group where it belongs. (And incidentally prove that the pass is enabled by default.)

[sebpop]

In D23136#505225, @probinson wrote:

Would you mind putting test/Feature/optnone-opt.ll back as it was before r276479? That would put the pass back in the group where it belongs. (And incidentally prove that the pass is enabled by default.)

Cherry-picked your previous patch in:
https://reviews.llvm.org/rL277688

Thanks for your review!

lib/Transforms/IPO/PassManagerBuilder.cpp

DisablePreInliner("disable-preinline", cl::init(false), cl::Hidden,
                       cl::desc("Disable pre-instrumentation inliner"));
 
 static cl::opt<int> PreInlineThreshold(
     "preinline-threshold", cl::Hidden, cl::init(75), cl::ZeroOrMore,
     cl::desc("Control the amount of inlining in pre-instrumentation inliner "
              "(default = 75)"));
 
 static cl::opt<bool> EnableGVNHoist(
-    "enable-gvn-hoist", cl::init(false), cl::Hidden,
-    cl::desc("Enable the experimental GVN Hoisting pass"));
+    "enable-gvn-hoist", cl::init(true), cl::Hidden,
+    cl::desc("Enable the experimental GVN Hoisting pass (default = on)"));
 
 PassManagerBuilder::PassManagerBuilder() {
     OptLevel = 2;
     SizeLevel = 0;
     LibraryInfo = nullptr;
     Inliner = nullptr;
     ModuleSummary = nullptr;
     DisableUnitAtATime = false;