cmd/compile/internal/gc: preallocate slices in noder

cmd/compile/internal/gc: preallocate slices in noder

The slice capacity is known for all these cases. Therefore,
we can initialize them with the desired capacity.

Change-Id: I1835b49108d157203d62e4aa119c2d7ab5e5e46f
Reviewed-on: https://go-review.googlesource.com/c/go/+/200119
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>