Consider adding a stress mode which prefers registers with different encoding characteristics

On various architectures there exist registers that require different encoding characteristics. For example, on x64 there are additional registers available (R8-R15 and XMM8-XMM15), which require the use of the REX prefix (or a VEX-encoded instruction).

Given that these change how the instruction is encoded, it would be beneficial if there was a stress-mode that would prefer these registers in order to help validate that the emitter has them properly handled.

category:testing theme:register-allocator skill-level:beginner cost:small