This reverts commit 7dd9c385.

Reason for revert: Reverting the revert, which will re-enable the convI2E optimization.  We originally reverted the convI2E optimization because it was making the builder fail, but the underlying cause was later determined to be unrelated.

Original CL: https://go-review.googlesource.com/31260
Revert CL: https://go-review.googlesource.com/31310
Real bug: https://go-review.googlesource.com/c/25159
Real fix: https://go-review.googlesource.com/c/31316

Change-Id: I17237bb577a23a7675a5caab970ccda71a4124f2
Reviewed-on: https://go-review.googlesource.com/32023
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>