cmd/compile: move remaining instrumentation logic into ssagen

cmd/compile: move remaining instrumentation logic into ssagen

Allows removing Func.{Enter,Exit}, which were only ever used to hold
at most one function call each.

Change-Id: I8b629c82e90bac3fcbe54db89900492406c7dca6
Reviewed-on: https://go-review.googlesource.com/c/go/+/528319
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>