cmd/compile: change phi location to be optimistic at backedges

This is:

(1) a simple trick that cuts the number of phi-nodes
(temporarily) inserted into the ssa representation by a factor
of 10, and can cut the user time to compile tricky inputs like
gogo/protobuf tests from 13 user minutes to 9.5, and memory
allocation from 3.4GB to 2.4GB.

(2) a fix to sparse lookup, that does not rely on
an assumption proven false by at least one pathological
input "etldlen".

These two changes fix unrelated compiler performance bugs,
both necessary to obtain good performance compiling etldlen.
Without them it takes 20 minutes or longer, with them it
completes in 2 minutes, without a gigantic memory footprint.

Updates #16407

Change-Id: Iaa8aaa8c706858b3d49de1c4865a7fd79e6f4ff7
Reviewed-on: https://go-review.googlesource.com/23136


Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>