JIT: Add a stress mode that poisons implicit byrefs (#80691)

JIT: Add a stress mode that poisons implicit byrefs (#80691)

This stress mode poisons all implicit byrefs before returns from the
method. GC pointers are nulled out and other parts of the structs are
filled with 0xcd bytes.

This should help expose incorrectly elided copies in the recently added
last-use copy elision optimization.