intel/fs/xehp: Teach SWSB pass about the exec pipeline of FS_OPCODE_PACK_HALF_2x16_SPLIT.

This virtual instruction is translated into multiple half float physical instructions, even though its destination is typically of integer type, which prevents the software scoreboard pass from inferring the correct execution pipeline for the virtual instruction on XeHP+ platforms. Teach the SWSB lowering pass about this inconsistency between the IR and physical instruction types.

Fixes among other tests:

dEQP-GLES31.functional.shaders.builtin_functions.pack_unpack.packhalf2x16_compute