[dev.regabi] cmd/compile: fix ir.Dump for []*CaseClause, etc

Dump uses reflection to print IR nodes, and it only knew how to print
out the Nodes slice type itself. This CL adds support for printing any
slice whose element type implements Node, such as SwitchStmt and
SelectStmt's clause lists.

Change-Id: I2fd8defe11868b564d1d389ea3cd9b8abcefac62
Reviewed-on: https://go-review.googlesource.com/c/go/+/281537


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>