go/ssa: simplify make([]T, len, k) to new([k]T)[:len] for constant k

+ test.

Change-Id: I0255e38a0f95af95a32dc1d9cf18b26db85af36f
Reviewed-on: https://go-review.googlesource.com/1519
Reviewed-by: Robert Griesemer <gri@golang.org>