[llvm] [llvm-exegesis] Add matcher for register initial values (PR #76666)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

I don't believe this matters for any functional use of the tool as the YAML parsing should only be needed in analysis mode which I don't believe depends on the initial register values, but it still seemed like a gap in the test coverage when I was working on YAML support for memory annotations.

https://github.com/llvm/llvm-project/pull/76666