Turns push_backs into emplace_backs and changes the TIntermSequence
constructor argument to &&.

Bug: angleproject:5535
Change-Id: I640ce879b6ade48a28dea6385ebb7a95cb8304ff
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2636680
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>