AMDGPU: Don't peel of the offset if the resulting base could possibly be...

AMDGPU: Don't peel of the offset if the resulting base could possibly be negative in Indirect addressing.

Summary:
  Don't peel of the offset if the resulting base could possibly be negative in Indirect addressing.
This is because the M0 field is of unsigned.

This patch achieves the similar goal as https://reviews.llvm.org/D55241, but keeps the optimization
if the base is known unsigned.

Reviewers:
  arsemn

Differential Revision:
  https://reviews.llvm.org/D55568

llvm-svn: 349951