The test runs far too long for -short mode (4 seconds).

Also removed useless test of now-disconnected knob
(GO_SSA_PHI_LOC_CUTOFF), which cuts 4 seconds to 2 seconds (which
is still too long), and finished removing the disconnected knob.

Updates #26469.

Change-Id: I6c594227c4a5aaffee46832049bdbbf570d86e60
Reviewed-on: https://go-review.googlesource.com/125075
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>