[InstSimplify] Fold inbounds GEP to poison if base is undef.

D92270 updated constant expression folding to fold inbounds GEP to
poison if the base is undef. Apply the same logic to SimplifyGEPInst.

The justification is that we can choose an out-of-bounds pointer as base
pointer.

Reviewed By: nikic, lebedev.ri

Differential Revision: https://reviews.llvm.org/D117015