Fix a bug compiling PR1978 (perhaps not the only one though) which

Fix a bug compiling PR1978 (perhaps not the only one though) which
was incorrectly simplifying "x == (gep x, 1, i)" into false, even
though i could be negative.  As it turns out, all the code to
handle this already existed, we just need to disable the incorrect
optimization case and let the general case handle it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46739 91177308-0d34-0410-b5e6-96231b3b80d8