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

[Namish Kukreja via llvm-commits]

namikukr wrote:

> 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.

I apologize - I'm a bit new to this so I'm just getting familiar. 

That seems accurate. I'm working on making the changes that were suggested and will be back with an update as soon as I have one.

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