[llvm] [llvm-exegesis] Set stack pointer register after starting perf counter (PR #72489)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> I'm wondering whether we even want to support user-defined values for the stack pointer anyway. Is there a use case for this ?

Yes. For the execution of arbitrary basic blocks with memory annotations for the construction of a datasets of BBs, there will definitely be BBs using the `%rsp` register (which is what we originally intended memory annotations to be used for). Not supporting this also prevents the use of instructions like `pushq` and `popq` in snippets which are bound to come up at some point in a microbenchmarking situation.

https://github.com/llvm/llvm-project/pull/72489