* x86 recipes: emit StackOverflow trap for all sp-relative loads and stores
* x86 recipes: emit StackOverflow trap for push and pop
* x86 binary filetests: add stk_ovf trap annotations
* Remove the mypy version constraint and set strict_optional to False.
* Add type annotations for `ISA` variables.
mypy 0.600 seems to require explicit annotations here.
* Annotate the ISA variables in the defs.py files too.
regmove, regfill, and regspill have immediates which aren't value
operands, so they aren't in the set of things that can be described by
the existing constraint system. Consequently, constraints saying that
the non-REX encodings only support registers that don't need REX
prefixes don't work. Fow now, just remove the non-REX encodings, so
that they don't get selected when they aren't valid.
This fixes the last known issue with instruction shrinking, so it can
be re-enabled.
