[powerpc] Assertion failure using llvm-mc -show-encoding with plxv

[efriedma-quic]

$ echo "plxv 34, x" | llvm-mc -triple=powerpc64 -show-encoding
llvm-mc: [...]/llvm/lib/MC/MCAsmStreamer.cpp:2390: void (anonymous namespace)::MCAsmStreamer::AddEncodingComment(const llvm::MCInst &, const llvm::MCSubtargetInfo &): Assertion `Bit == 0 && "Encoder wrote into fixed up bit!"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.

[llvmbot]

@llvm/issue-subscribers-backend-powerpc

Author: Eli Friedman (efriedma-quic)

``` $ echo "plxv 34, x" | llvm-mc -triple=powerpc64 -show-encoding llvm-mc: [...]/llvm/lib/MC/MCAsmStreamer.cpp:2390: void (anonymous namespace)::MCAsmStreamer::AddEncodingComment(const llvm::MCInst &, const llvm::MCSubtargetInfo &): Assertion `Bit == 0 && "Encoder wrote into fixed up bit!"' failed. PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace. ```

[nemanjai]

What should happen here? The instruction asm string is invalid. I suppose it would be preferable to emit a more friendly diagnostic. We get a similar (but different) assert failure with plxv 34, x(11) which is also obviously invalid.

[efriedma-quic]

I mostly expect it to not crash, yes, hopefully with some hint for what went wrong.