intel/fs: Add an ex_desc field to fs_inst for SHADER_OPCODE_SEND

I meant to do this years ago when I first added SHADER_OPCODE_SEND.  At
the time, the only use for the extended descriptor was bindless handles
which were always one thing and never non-constant.  However, it doesn't
actually require any extra instructions because we have to OR in ex_mlen
anyway.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8748>

src/intel/compiler/brw_ir.h

@@ -152,6 +152,7 @@ struct backend_instruction {
    uint8_t target; /**< MRT target. */
    uint8_t sfid; /**< SFID for SEND instructions */
    uint32_t desc; /**< SEND[S] message descriptor immediate */
+   uint32_t ex_desc; /**< SEND[S] extended message descriptor immediate */
    unsigned size_written; /**< Data written to the destination register in bytes. */
 
    enum opcode opcode; /* BRW_OPCODE_* or FS_OPCODE_* */