Rename global-merge to enable-global-merge.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210639 91177308-0d34-0410-b5e6-96231b3b80d8
Global merge for global symbols.

This commit is to improve global merge pass and support global symbol merge.
The global symbol merge is not enabled by default. For aarch64, we need some
more back-end fix to make it really benifit ADRP CSE.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210640 91177308-0d34-0410-b5e6-96231b3b80d8