AMDGPU: Fix incorrect simm check

Use signed division otherwise all back jumps fail the check
Fixes regression introduced in r269951

Differential Revision: http://reviews.llvm.org/D20380

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@269972 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed