Fix implicit null check with negative offset

Summary:
It seems that negative offset was accidentally allowed in D17967.
AFAICT small negative offset should be valid (always raise segfault) on all archs that I'm aware of (especially x86, which is the only one with this optimization enabled) and such case can be useful when loading hiden metadata from an object.

However, like the positive side, it should only be done within a certain limit.
For now, use the same limit on the positive side for the negative side.
A separate option can be added if needs appear.

Reviewers: mcrosier, skatkov

Reviewed By: skatkov

Subscribers: sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D38925

llvm-svn: 315991