[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.

llvm-svn: 234666

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