[mux] Remove degenerate subgroup support. #564

Job	Run time
Call PR testing on schedule / run_host_x86_64	11m 10s
Call PR testing on schedule / run_riscv_m1	10m 18s
Call PR testing on schedule / run_clang_format	10s
Call PR testing on schedule / run_clang_tidy_changes	8m 29s
Call PR testing on schedule / run_windows_msvc_x86_64_llvm_latest_cl3_0_offline	8m 45s
Call PR testing on schedule / run_ubuntu_gcc_x86_64_riscv_fp16_cl3_0_unitcl_vecz	8m 18s
Call PR testing on schedule / run-ubuntu-clang-x86-llvm-latest-cl3-0-offline	8m 17s
Call PR testing on schedule / run-ubuntu-gcc-aarch64-llvm-latest-cl3-0-fp16	7m 50s
Call PR testing on schedule / run-ubuntu-gcc-x86_64-clik-refsi	2m 58s
Call PR testing on schedule / run-ubuntu-gcc-x86_64-riscv-fp16-cl3-0	8m 15s
Call PR testing on schedule / run-ubuntu-gcc-x86-llvm-latest-x86_64-images-cl3-0-release	8m 15s
Call PR testing on schedule / run-ubuntu-gcc-x86_64-refsi-g1-wi-cl3-0	0s
	1h 22m 45s

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