R600/SI: return undef instead of null for skipped arguments

This is a candidate for the stable branch.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=64694

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182084 91177308-0d34-0410-b5e6-96231b3b80d8