[llvm] [update_mc_test_checks] Support --show-inst output (PR #174011)

[Alexander Richardson via llvm-commits]

arichardson wrote:

> @arichardson, your change seems to be failing on machines with python3.8 installed, can you take a look?
> 
> ```
> $ python3 --version
> Python 3.8.10
> $ python3 update_mc_test_checks.py
> Traceback (most recent call last):
>   File "update_mc_test_checks.py", line 115, in <module>
>     def getStdCheckLines(prefix: str, output: str, mc_mode) -> list[str]:
> TypeError: 'type' object is not subscriptable
> ```
> 
> This is also causing failures on 2 bots that run on Ubuntu 20.04 LTS which has python 3.8 as the latest version:
> 
> * https://lab.llvm.org/buildbot/#/builders/174/builds/30327
> * https://lab.llvm.org/buildbot/#/builders/202/builds/5103

Should be fixed by https://github.com/llvm/llvm-project/pull/176978 which I just merged. Please let me know if it's still broken

https://github.com/llvm/llvm-project/pull/174011