2021.09.03 tests with pip version of angr:

   * avx gives warning-unrollerror with clang -O1 and gcc -O3 since
     Iop_QNarrowBin16Sto8Sx16 isn't supported in angr.

   * ref gives warning-unrollerror with gcc -O3 since
     Iop_QNarrowBin32Sto16Ux8 isn't supported in angr.

   * ref with clang -O1 works fine, but of course isn't sufficient for
     avx-vs.-ref tests.

2021.09.04 tests with angr-dev: Unrolling now works fine thanks to
https://github.com/angr/angr/pull/2887. avx=ref with both compilers,
ref=ref across compilers.

2021.09.17 benchmarks, EPYC 7501:

build/encode_761xfreeze3/ref/gcc_-O3_-march_native_-mtune_native equals-ref-clang_-O1_-fwrapv_-march_native
build/encode_761xfreeze3/ref/gcc_-O3_-march_native_-mtune_native seconds equiv 18878.019161
build/encode_761xfreeze3/avx/gcc_-O3_-march_native_-mtune_native equals-ref-gcc_-O3_-march_native_-mtune_native
build/encode_761xfreeze3/avx/gcc_-O3_-march_native_-mtune_native seconds equiv 75394.262480
build/encode_761xfreeze3/avx/clang_-O1_-fwrapv_-march_native equals-ref-clang_-O1_-fwrapv_-march_native
build/encode_761xfreeze3/avx/clang_-O1_-fwrapv_-march_native seconds equiv 162381.331008