AMDGPU/R600: Fix GlobalValue regressions.

Don't cast GV expression to MCSymbolRefExpr. r272705 changed GV to binary
expressions by including offset even if the offset it 0
(we haven't hit this sooner since tested workloads don't include static offsets)
We don't really care about the type of expression, so set it directly.
Fixes: r272705

Consider section relative relocations. Since all const as data is in one boffer section relative is equivalent to abs32.
Fixes: r273166

Differential Revision: http://reviews.llvm.org/D21633

llvm-svn: 273785