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

[Simon Wallis via llvm-commits]

simonwallis2 wrote:

I see little benefit in adding new tests for this commit.
There is reasonable coverage already from llvm-project/llvm/test/tools/llvm-exegesis/AArch64/all-opcodes.test

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