freedreno/a6xx: Move assert

The flags_regid is only present if gs is present (in which case, gs is
the last_shader).  If there is no gs, flags_regid is initialized to
zero, not INVALID_REG (r63.x).  But you have to scroll up several pages
of a long fxn to see that.

Move the assert to make things more clear.

Signed-off-by: Rob Clark <rob.clark@oss.qualcomm.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39029>

src/gallium/drivers/freedreno/a6xx/fd6_program.cc

@@ -882,9 +882,6 @@ emit_vpc(fd_crb &crb, const struct program_builder *b)
       }
    }
 
-   /* if vertex_flags somehow gets optimized out, your gonna have a bad time: */
-   assert(flags_regid != INVALID_REG);
-
    switch (last_shader->type) {
    case MESA_SHADER_VERTEX:
       crb.add(A6XX_SP_VS_OUTPUT_CNTL(.out = linkage.cnt));
@@ -925,6 +922,9 @@ emit_vpc(fd_crb &crb, const struct program_builder *b)
       ));
       break;
    case MESA_SHADER_GEOMETRY:
+      /* if vertex_flags somehow gets optimized out, your gonna have a bad time: */
+      assert(flags_regid != INVALID_REG);
+
       crb.add(A6XX_SP_GS_OUTPUT_CNTL(.out = linkage.cnt, .flags_regid = flags_regid));
       crb.add(VPC_GS_SIV_CNTL(CHIP,
          .layerloc = layer_loc,