cmd/compile: improve ir.StaticValue and extract ir.StaticCalleeName

cmd/compile: improve ir.StaticValue and extract ir.StaticCalleeName

This CL extends ir.StaticValue to also work on closure variables.

Also, it extracts the code from escape analysis that's responsible for
determining the static callee of a function. This will be useful when
go/defer statement normalization is moved to typecheck.

Change-Id: I69e1f7fb185658dc9fbfdc69d0f511c84df1d3ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/518959
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>