Global variables with strong definitions cannot be freed

With the current deref semantics, this is redundant - since we assume that anything which is dereferenceable (ever) can't be freed - but it becomes neccessary for the deref-at-point semantics.

Testing wise, this is covered by test/CodeGen/X86/hoist-invariant-load.ll when -use-dereferenceable-at-point-semantics is active.  I didn't bother duplicating the command line since a) it's an in-development mode, and b) the change is pretty obvious.