AMDGPU: change a redundant if () to an assert(). NFC

Summary:
I've been carrying this change around with me for a while, because the if ()
managed to confuse me while following the code. All callers ensure that the
assertion holds.

Reviewers: arsenm, tstellarAMD

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D19042

llvm-svn: 266344