[llvm] [llvm-exegesis] Add AArch64 operand initializers, SetRegTo (PR #169912)

[Sjoerd Meijer via llvm-commits]

sjoerdmeijer wrote:

> Please get someone who knows the ARM side of things well to approve that side of things before landing.

I would like to look at tests that show these instructions printed (preview assembly) or disassembled (dump obj to disk) so there's different ways. Not only is it easier to read and check, having tests is probably better. I quite like the tests in `llvm/test/tools/llvm-exegesis/AArch64/setReg_init_check.s` because it doesn't depend on a debug build like the ones in `debug-gen-asm.s`.

https://github.com/llvm/llvm-project/pull/169912