Fixes glean fragProg1 regression caused by commit b9f68d92


(implement TGSI_PROPERTY_FS_COLOR0_WRITES_ALL_CBUFS).  This bug
only appears when the fragment shader emits fragment.Z before
color outputs.  The bug was caused by confusion between register
indexes and semantic indexes.

Also added some comments to better explain register indexing.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>