[llvm] [llvm-mca] Abort on parse error without -skip-unsupported-instructions (PR #90474)

[Peter Waller via llvm-commits]

peterwaller-arm wrote:

> Could you please add a brief description of flag -skip-unsupported-instructions to the llvm-mca command guide?

Done. Text reads:

```
.. option:: -skip-unsupported-instructions

  Force :program:`llvm-mca` to continue even in the presence of instructions
  which do not parse or lack key scheduling formation. Note that the resulting
  analysis is impacted since those unsupported instructions are ignored as-if
  they are not supplied as a part of the input.
```

https://github.com/llvm/llvm-project/pull/90474