… transformation

Functions with GS cookie checks may furthermore shadow parameters to
make sure that the GS cookie ends up above all parameters on the stack.
This is done by creating a copy of parameters used like a pointer and
redirection all uses to act on the shadow. IR is inserted in the
beginning of the function to copy the parameters to their shadows.

For async this introduces a problem when it comes to implicit byrefs.
The shadowing ceremony introduces a pointer to a local (the storage area
in the caller) and that pointer remains live across suspension points,
resulting in illegal IR.

Ideally we would move the shadowing pass to run after the async
transformation, but this requires rewriting the analysis from HIR to
LIR. This change instead fixes the issue by keeping the analysis where
it is, but by delaying the rewrite of the IR until after the async
transformation (for async functions only).