[GlobalOpt] Allow constant globals to be SRA'd

[GlobalOpt] Allow constant globals to be SRA'd

The current logic assumes that any constant global will never be SRA'd. I presume this is because normally constant globals can be pushed into their uses and deleted. However, that sometimes can't happen (which is where you really want SRA, so the elements that can be eliminated, are!).

There seems to be no reason why we can't SRA constants too, so let's do it.

llvm-svn: 267393