R600/SI: Allow SI_KILL for geometry shaders

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202336 91177308-0d34-0410-b5e6-96231b3b80d8
R600/SI: Optimize SI_KILL for constant operands

If the SI_KILL operand is constant, we can either clear the exec mask if
the operand is negative, or do nothing otherwise.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202337 91177308-0d34-0410-b5e6-96231b3b80d8