[CodeGen] Split -enable-global-merge into ARM and AArch64 options.

Currently, there's a single flag, checked by the pass itself.
It can't force-enable the pass (and is on by default), because it
might not even have been created, as that's the targets decision.
Instead, have separate explicit flags, so that the decision is
consistently made in the target.

Keep the flag as a last-resort "force-disable GlobalMerge" for now,
for backwards compatibility.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234666 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/GlobalMerge.cpp

@@ -73,9 +73,10 @@ using namespace llvm;
 
 #define DEBUG_TYPE "global-merge"
 
+// FIXME: This is only useful as a last-resort way to disable the pass.
 static cl::opt<bool>
 EnableGlobalMerge("enable-global-merge", cl::Hidden,
-                  cl::desc("Enable global merge pass"),
+                  cl::desc("Enable the global merge pass"),
                   cl::init(true));
 
 static cl::opt<bool>