Fix regression from my recent GlobalsAA fix.

There are two fixes here: one, AnalyzeUsesOfPointer can't return
false until it has checked all the uses of the pointer. Two, if a
global uses another global, we have to assume the address of the
first global escapes.

Fixes https://llvm.org/bugs/show_bug.cgi?id=30707 .

Differential Revision: https://reviews.llvm.org/D25798

llvm-svn: 285034