spirv: disable mediump for bitfield insert/extract

This prevents a regression with the next change that generalizes
bitfield insert/extract sizes.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35209>

src/compiler/spirv/vtn_alu.c

@@ -585,6 +585,10 @@ vtn_alu_op_mediump_16bit(struct vtn_builder *b, SpvOp opcode, struct vtn_value *
    case SpvOpFwidthFine:
    case SpvOpFwidthCoarse:
       return b->options->mediump_16bit_derivatives;
+   case SpvOpBitFieldInsert:
+   case SpvOpBitFieldSExtract:
+   case SpvOpBitFieldUExtract:
+      return false;
    default:
       return true;
    }