R600/SI: Clean up checks for legality of immediate operands

There are new register classes VCSrc_* which represent operands that
can take an SGPR, VGPR or inline constant.  The VSrc_* class is now used
to represent operands that can take an SGPR, VGPR, or a 32-bit
immediate.

This allows us to have more accurate checks for legality of
immediates, since before we had no way to distinguish between operands
that supported any 32-bit immediate and operands which could only
support inline constants.