[llvm] [SystemZ] Add check for INIT_UNDEF in getInstSizeInBytes (PR #134661)

[via llvm-commits]

tltao wrote:

> We probably should be more generic here and check for `isMetaInstruction`, which would also cover some of the existing special cases. However, `INIT_UNDEF` isn't currently even marked as `isMeta` - probably it should be, though.

I believe @nikic tried this in the original PR https://github.com/llvm/llvm-project/pull/106744, but ultimately decided against it as it caused more changes. Perhaps a compromise can be to fix it with `MI.getOpcode() == TargetOpcode::INIT_UNDEF` for now?

https://github.com/llvm/llvm-project/pull/134661