Rewrite the global merge pass to be subprogram agnostic for now.

Rewrite the global merge pass to be subprogram agnostic for now.
It was previously using the subtarget to get values for the global
offset without actually checking each function as it was generating
code. Go ahead and solidify the current behavior and make the
existing FIXMEs more prominent.

As a note the ARM backend previously had a thumb1 and non-thumb1
set of defaults. Only the former was tested so I've changed the
behavior to only use that for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230245 91177308-0d34-0410-b5e6-96231b3b80d8