IR: Do not canonicalize constant GEPs into an out-of-bounds array access

Summary:
Consider a GEP of:
i8* getelementptr ({ [2 x i8], i32, i8, [3 x i8] }* @main.c, i32 0, i32 0, i64 0)

If we proceeded to GEP the aforementioned object by 8, would form a GEP of:
i8* getelementptr ({ [2 x i8], i32, i8, [3 x i8] }* @main.c, i32 0, i32 0, i64 8)

Note that we would go through the first array member, causing an
out-of-bounds accesses.  This is problematic because we might get fooled
if we are trying to evaluate loads using this GEP, for example, based
off of an object with a constant initializer where the array is zero.

This fixes PR17732.

Reviewers: nicholas, chandlerc, void

Reviewed By: void

CC: llvm-commits, echristo, void, aemerson

Differential Revision: http://llvm-reviews.chandlerc.com/D2093