[llvm] Add metadata pointer regex substitution to MIR update script (PR #163253)

[Joe Nash via llvm-commits]

Sisyph wrote:

> @davemgreen @MatzeB If either of you could give this a quick review that would be great

I will add them as reviewers. I forgot you need commit access to do that.

https://github.com/llvm/llvm-project/pull/163253