r600g/sb: fix handling of PS in source bytecode on cayman

Actually PS doesn't make sense for cayman and isn't even mentioned in
cayman docs, but llvm backend currently uses it in bytecode and, assuming
that hw seems to be mostly ok with it, this will allow sb to parse such
source bytecode correctly.

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>

src/gallium/drivers/r600/sb/sb_bc_parser.cpp

@@ -385,6 +385,11 @@ int bc_parser::prepare_alu_group(cf_node* cf, alu_group_node *g) {
 			} else if (src.sel == ALU_SRC_PS || src.sel == ALU_SRC_PV) {
 				unsigned pgroup = !cgroup, prev_slot = src.sel == ALU_SRC_PS ?
 						SLOT_TRANS : src.chan;
+
+				// XXX shouldn't happen but llvm backend uses PS on cayman
+				if (prev_slot == SLOT_TRANS && ctx.is_cayman())
+					prev_slot = SLOT_X;
+
 				alu_node *prev_alu = slots[pgroup][prev_slot];
 
 				assert(prev_alu);