Clean up RVV register composition. #241

Job	Run time
build-native-gcc	5m 48s
build-native-llvm	3m 44s
test-native-gcc	9m 8s
test-native-llvm	8m 41s
build-aarch64-gcc	4m 1s
build-aarch64-llvm	6m 31s
build-armhf-gcc	4m 39s
build-armhf-llvm	1m 50s
build-ppc64el-gcc	5m 49s
build-ppc64el-llvm	3m 23s
build-s390x-gcc	2m 24s
build-s390x-llvm	5m 9s
build-riscv64-llvm	1m 55s
test-aarch64-gcc (qemu_cpu: "max,sve=off")	18m 38s
test-aarch64-gcc (qemu_cpu: "max,sve=on,sve128=on")	22m 47s
test-aarch64-gcc (qemu_cpu: "max,sve=on,sve256=on")	29m 31s
test-aarch64-gcc (qemu_cpu: "max,sve=on,sve512=on")	31m 11s
test-aarch64-llvm (qemu_cpu: "max,sve=off")	16m 16s
test-aarch64-llvm (qemu_cpu: "max,sve=on,sve128=on")	20m 50s
test-aarch64-llvm (qemu_cpu: "max,sve=on,sve256=on")	25m 49s
test-aarch64-llvm (qemu_cpu: "max,sve=on,sve512=on")	31m 36s
test-armhf-gcc (qemu_cpu: "max")	14m 33s
test-armhf-llvm (qemu_cpu: "max")	15m 54s
test-ppc64el-gcc (qemu_cpu: "power10")	29m 33s
test-ppc64el-llvm (qemu_cpu: "power10")	27m 54s
test-s390x-gcc (qemu_cpu: "")	22m 47s
test-s390x-llvm (qemu_cpu: "")	19m 35s
test-riscv64-llvm (qemu_cpu: "rv64,zba=true,zbb=true,zbs=true,v=false")	7m 24s
test-riscv64-llvm (qemu_cpu: "rv64,zba=true,zbb=true,zbs=true,v=true,vlen=128,elen=64,vext_spec=v1.0")	15m 16s
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")	32m 16s
	7h 45m 31s

Provide feedback