Avoid DCE at Go SSA level

[aykevl]

Large change that:

1. Adds support for calling indirect functions, like the following (as used in the time package):

   func foo(fn func()) {
       go fn()
   }

2. Compiles all code, instead of just the part that seems to be used.

This is in preparation of fixing #285. Not ready for merging, as it's not complete (some packages that previously compiled break with this change) and I want to review/improve it myself a bit, and add real commit messages.

This change will increase compile times, but should decrease massively once #285 is ready and package builds can be cached.

[aykevl]

Progress! After rebasing to incorporate the changes in #486, the amount of successful checks goes from 1 to 3 (of 4). And I know what goes wrong in the 4th one (test-llvm8-go111).

[aykevl]

The main limitation now is that code like the following causes a compiler error:

package main

type asdf struct {
	next []asdf
}

func main() {
	foo(new(asdf))
}

func foo(x *asdf) {
}

(The issue here is the struct that contains a slice of itself).
This issue is present in the strings package, which worked before and should definitely not regress with this PR.

[aykevl]

Replaced by #1008.