cmd/compile: PGO devirtualization of closures

[prattmic]

https://go.dev/cl/539699 adds PGO devirtualization of calls to function values, but it is limited to callees that are standard functions. Closures (any function requiring closure context) are excluded due to the extra complexity of passing the closure context.

It should be feasible to devirtualize these calls, but will require extra work in SSA to pass a closure context to a closure call as well as work in IR to represent such static closure calls.

cc @cherrymui @aclements @mdempsky