[llvm] [ARM64EC] Warn on using disallowed registers in assembly src. (PR #93618)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> > Thanks, this looks very valuable!
> > Can you mention how to enable this warning when using armasm64? (Or I guess it probably gets enabled automatically if assembling with some flag to switch to arm64ec mode - what's the armasm64 flag for that?)
> 
> When assembling with `cl` on Windows, the flag is `-arm64EC`. It's mirrored similarly by `clang-cl`. I'll include that in the commit message.

But you mentioned `armasm64` - one doesn't build assembly with `cl`. But I looked it up and tested myself - to assemble arm64ec object files with `armasm64`, one passes the flags `-machine arm64ec`, and then it does automatically warn about use of forbidden registers.

https://github.com/llvm/llvm-project/pull/93618