R600/SI: Fix int_SI_fs_interp_constant

The important fix is that the constant interpolation value is stored in the
parameter slot P0, which is encoded as 2.

In addition, drop the SI_INTERP_CONST pseudo instruction, pass the parameter
slot as an operand to V_INTERP_MOV_F32 instead of hardcoding it there, and
add a special operand class for the parameter slots for type checking and
pretty printing.

NOTE: This is a candidate for the Mesa stable branch.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>