This implements the equivalent of 07443e2d41 in Jit64 and JitArm64.
Aims to fix https://bugs.dolphin-emu.org/issues/13865.

By moving members of the OpArg struct around, we can cut down on how
much padding the struct needs. Now it has a size of 16 bytes, small
enough for function calls to pass it in two registers instead of on the
stack.

Conditional jumps already do that, so let's be consistent.

LLVM does this, so let's do it as well.

The inequality operator is automatically generated by the compiler if `operator==` is defined.

xEmitter: Convert PrefetchLevel to enum class

This should prevent future occurrences of the type of problem that was
fixed in b713042.