Make X64Assembler short-lived
This is a wide-ranging refactor. We should not have long-lived (i.e.
member) X64Assemblers anymore.
A couple of these changes are non-mechanical, and I'll point them out in
inline comments.
There are a couple things left to do:
- CodeGenerator still has member assemblers. We'll need to apply similar
changes there, and it's just a ton of functions. I'm leaving that for
a separate diff.
- This gets us closer to being able to remove the frontier(), base(),
etc. member functions from X64Assembler, which will be the forcing
function to finally separate CodeBlock concerns from assembler
concerns.
Differential Revision:
D950811