Also hash predicates by address

As suggested in PR feedback:

https://github.com/rust-lang/rust/pull/73011#discussion_r435728923

This allows count_code_region() to be handled like a normal intrinsic so
the InstanceDef::InjectedCode variant is no longer needed.

This initial version only injects counters at the top of each function.
Rust Coverage will require injecting additional counters at each
conditional code branch.

the need to evaluate consts eagerly and therefore gets around const eval query cycles.

asm! is left as a wrapper around llvm_asm! to maintain compatibility.