Investigate removing CmpNeNull bytecode

We can replace

CmpNeNull
JmpTrue (JmpFalse)

with

CmpEqNull
JmpFalse (JmpTrue) 

without any overhead. Any context where we actually want to store the value can emit an additinoal Neg instruction, which should have negligible overhead.

Any strict comparison against null or undefined will end up calling CmpStrictNotEqual
in any case.