[AMDGPU][MC][gfx8] Support 20-bit immediate offset in SMEM instructions.

Fixes Bug 30808.
Note that passing subtarget information to predicates seems too complicated, so gfx8-specific def smrd_offset_20 introduced.
Old gfx6/7-specific def renamed to smrd_offset_8 for clarity.
Lit tests updated.

Differential Revision: https://reviews.llvm.org/D26085

llvm-svn: 285590