pan/midgard: avoid implicit cast-warning on Clang

BITFIELD_MASK() returns a 32-bit unsigned integer, and Clang complains
if we assign it to a 16-bit unsigned integer without a cast. Let's add
that cast.

While we're at it, add an assert() to make it clear to the compiler that
the condition in BITFIELD_MASK() can be optimized away.

Reviewed-by: Yiwei Zhang <zzyiwei@chromium.org>
Tested-by: Yiwei Zhang <zzyiwei@chromium.org>
Reviewed-by: Eric R. Smith <eric.smith@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36606>

src/panfrost/midgard/midgard_compile.c

@@ -1642,7 +1642,8 @@ emit_intrinsic(compiler_context *ctx, nir_intrinsic_instr *instr)
 
       ins.dest_type = ins.src_types[1] = nir_type_uint | instr->def.bit_size;
 
-      ins.mask = BITFIELD_MASK(instr->def.num_components);
+      assert(instr->def.num_components <= 4);
+      ins.mask = (uint16_t)BITFIELD_MASK(instr->def.num_components);
       emit_mir_instruction(ctx, &ins);
       break;
    }