[GMR] Teach the conservative path of GMR to catch even more easy cases.

In PR24288 it was pointed out that the easy case of a non-escaping
global and something that *obviously* required an escape sometimes is
hidden behind PHIs (or selects in theory). Because we have this binary
test, we can easily just check that all possible input values satisfy
the requirement. This is done with a (very small) recursion through PHIs
and selects. With this, the specific example from the PR is correctly
folded by GVN.

Differential Revision: http://reviews.llvm.org/D11707

llvm-svn: 244078