i965/vec4: Take into account non-zero reg_offset during register allocation.

Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/drivers/dri/i965/brw_vec4_reg_allocate.cpp

@@ -38,7 +38,9 @@ static void
 assign(unsigned int *reg_hw_locations, backend_reg *reg)
 {
    if (reg->file == GRF) {
-      reg->reg = reg_hw_locations[reg->reg];
+      assert(reg->reg_offset >= 0);
+      reg->reg = reg_hw_locations[reg->reg] + reg->reg_offset;
+      reg->reg_offset = 0;
    }
 }