[AMDGPU] Removed dead code from R600ISelLowering.cpp

This was added to inhibit a warning from gcc 7.3 according to
the comment. However, it triggers warning from PVS. In addition
I cannot reproduce it with gcc 7.4 and I also cannot reproduce
it with gcc 7.3 using compiler explorer.

Differential Revision: https://reviews.llvm.org/D69863