AMDGPU: Add feature vi-insts

This is necessary to add a VI specific builtin,
__builtin_amdgcn_s_dcache_wb. We already have an
overly specific feature for one of these builtins,
for s_memrealtime. I'm not sure whether it's better
to add more of those, or to get rid of that and merge
it with vi-insts.

Alternatively, maybe this logically goes with scalar-stores?

llvm-svn: 339104