Clean up RVV register composition. #235

Job	Run time
build-native-gcc	5m 43s
build-native-llvm	3m 45s
test-native-gcc	12m 35s
test-native-llvm	7m 27s
build-aarch64-gcc	9m 19s
build-aarch64-llvm	4m 59s
build-armhf-gcc	7m 12s
build-armhf-llvm	6m 57s
build-ppc64el-gcc	5m 52s
build-ppc64el-llvm	9m 22s
build-s390x-gcc	4m 10s
build-s390x-llvm	8m 11s
build-riscv64-llvm	3m 25s
test-aarch64-gcc (qemu_cpu: "max,sve=off")	17m 0s
test-aarch64-gcc (qemu_cpu: "max,sve=on,sve128=on")	24m 24s
test-aarch64-gcc (qemu_cpu: "max,sve=on,sve256=on")	29m 48s
test-aarch64-gcc (qemu_cpu: "max,sve=on,sve512=on")	30m 17s
test-aarch64-llvm (qemu_cpu: "max,sve=off")	19m 51s
test-aarch64-llvm (qemu_cpu: "max,sve=on,sve128=on")	20m 43s
test-aarch64-llvm (qemu_cpu: "max,sve=on,sve256=on")	28m 46s
test-aarch64-llvm (qemu_cpu: "max,sve=on,sve512=on")	28m 41s
test-armhf-gcc (qemu_cpu: "max")	17m 24s
test-armhf-llvm (qemu_cpu: "max")	13m 31s
test-ppc64el-gcc (qemu_cpu: "power10")	26m 39s
test-ppc64el-llvm (qemu_cpu: "power10")	26m 47s
test-s390x-gcc (qemu_cpu: "")	19m 51s
test-s390x-llvm (qemu_cpu: "")	21m 35s
test-riscv64-llvm (qemu_cpu: "rv64,zba=true,zbb=true,zbs=true,v=false")	4m 18s
test-riscv64-llvm (qemu_cpu: "rv64,zba=true,zbb=true,zbs=true,v=true,vlen=128,elen=64,vext_spec=v1.0")	13m 51s
test-riscv64-llvm (qemu_cpu: "rv64,zba=true,zbb=true,zbs=true,v=true,vlen=256,elen=64,vext_spec=v1.0")	20m 39s
test-riscv64-llvm (qemu_cpu: "rv64,zba=true,zbb=true,zbs=true,v=true,vlen=512,elen=64,vext_spec=v1.0")	34m 15s
	8h 7m 17s

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