cmd/compile: use int for field index

cmd/compile: use int for field index

All of a struct's fields have to fit into memory anyway, so index them
with int instead of int64.  This also makes it nicer for
cmd/compile/internal/gc to reuse the same NumFields function.

Change-Id: I210be804a0c33370ec9977414918c02c675b0fbe
Reviewed-on: https://go-review.googlesource.com/20691
Reviewed-by: Ian Lance Taylor <iant@golang.org>