[dev.regabi] cmd/compile: change nowritebarrierrec to use map[*ir.Func]

[dev.regabi] cmd/compile: change nowritebarrierrec to use map[*ir.Func]

All of the uses were already using *ir.Func index operands, so only
needs the map type itself updated.

Passes buildall w/ toolstash -cmp.

Updates #42982.

Change-Id: I568d8601f3eb077e07e887f2071aa1a2667d803c
Reviewed-on: https://go-review.googlesource.com/c/go/+/275754
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>