cmd/compile: call fninit earlier

This allows the global initializers function to go through normal
mid-end optimizations (e.g., inlining, escape analysis) like any other
function.

Updates #33485.

Change-Id: I9bcfe98b8628d1aca09b4c238d8d3b74c69010a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/254839


Reviewed-by: Keith Randall <khr@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>