cmd/compile/internal/inline: refactor heuristics analyzers

cmd/compile/internal/inline: refactor heuristics analyzers

This very minor refactoring changes the heuristics analysis code to
avoid running result-flag or param-flag analyzers on functions that
don't have any interesting results or parameters (so as to save a bit
of compile time). No change otherwise in heuristics functionality.

Change-Id: I7ee13f0499cc3d14d5638e2193e4bd8d7b690e5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/537976
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>