cmd/compile: PGO devirtualization follow-ups

[prattmic]

https://go.dev/cl/492436 implements the bare minimum of a PGO devirtualization implementation. There are still numerous limitations that we would like address shortly:

• Callees not directly referenced in the current package can be missed (even if they are in the transitive dependences). https://go.dev/cl/497175
• Only interface method calls are supported, not other indirect function calls.
• Callees that only partially implement an interface (they are embedded in another type that completes the interface) cannot be devirtualized. -> cmd/compile: PGO devirtualization of partial interface implementations #64673
• Closures cannot be inlined -> cmd/compile: PGO devirtualization of closures #64675
• Callees may be missing from export data -> cmd/compile: include PGO devirtualization targets in export data #64676

cc @cherrymui @aclements

[gopherbot]

Change https://go.dev/cl/497175 mentions this issue: cmd/compile: lookup indirect callees from export data for devirtualization

[gopherbot]

Change https://go.dev/cl/529557 mentions this issue: cmd/internal/objabi: add inverse of PathToPrefix

[gopherbot]

Change https://go.dev/cl/539698 mentions this issue: cmd/compile: move FuncPC intrinsic handling to common helper

[gopherbot]

Change https://go.dev/cl/539699 mentions this issue: cmd/compile: initial function value devirtualization

[gopherbot]

Change https://go.dev/cl/540258 mentions this issue: cmd/compile: support lookup of functions from export data