Implement global merge optimization for global variables.

This commit implements two command line switches -global-merge-on-external
and -global-merge-aligned, and both of them are false by default, so this
optimization is disabled by default for all targets.

For ARM64, some back-end behaviors need to be tuned to get this optimization
further enabled.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208934 91177308-0d34-0410-b5e6-96231b3b80d8