go/ssa: opt: avoid an allocation

In the dom tree traversal, the final child of each node now
inherits the parent's renaming map, reducing garbage.

This reduces allocations by 1.4% and bytes allocated by 2.0% when
building SSA for the entire standard library.

Change-Id: Id19b6d6766b3e0bf32d1db1238eff8a42d11b242
Reviewed-on: https://go-review.googlesource.com/45833
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
1 file changed