Remove hardcoded registers from the pass

[OleksiiOleksenko]

Some of the registers used in the instrumentation are hardcoded (e.g., R15 in X86SpecFuzzPass::visitWrite). This approach works, but it does not produce an optimal assembly because we have to push/pop the register value every time we use it.

A better approach would be to look for non-live registers and, if available, use them.