Does LLVM's bigint implementation use those instructions? Last I tried LLVM can not handle the required data-dependencies on individual carry bits, making it impossible to use ADCX/ADOX in LLVM without inline assembly.
For the iXYZ themselves, the carries are properly handled and it can generate mulx at least: https://github.com/herumi/mcl/blob/master/src/asm/x86-64.bmi... (from straight LLVM IR). I don't think ADCX/ADOX are possible though even in LLVM IR.
I think you are mixing with one comment on the GCC mailing list about GCC not having the adequate representation of carry and even less a representation that can separate a carry and overflow chains[2]
[1] https://gcc.godbolt.org/z/2h768y [2] https://gcc.gnu.org/legacy-ml/gcc-help/2017-08/msg00100.html
I was hoping LLVM could maybe bypass this IR limitation for the iXYZ types and generate the optimal instruction sequence, but from your assembly it looks like that doesn't happen either.