cmd/compile: make setting and accessing of node slice elements more uniform

cmd/compile: make setting and accessing of node slice elements more uniform

Add Set3 function to complement existing Set1 and Set2 functions.
Consistently use Set1, Set2 and Set3 for []*Node instead of Set where applicable.

Add SetFirst and SetSecond for setting elements of []*Node to mirror
First and Second for accessing elements in []*Node.

Replace uses of Index by First and Second and
SetIndex with SetFirst and SetSecond where applicable.

Passes toolstash -cmp.

Change-Id: I8255aae768cf245c8f93eec2e9efa05b8112b4e5
Reviewed-on: https://go-review.googlesource.com/37430
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>