cmd/compile: remove state.exitCode

We're holding onto the function's complete AST anyway, so might as
well grab the exit code from there.

Passes toolstash-check.

Change-Id: I851b5dfdb53f991e9cd9488d25d0d2abc2a8379f
Reviewed-on: https://go-review.googlesource.com/99417


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>