[dev.ssa] cmd/compile/ssa: don't nil check phis with non-nil arguments

[dev.ssa] cmd/compile/ssa: don't nil check phis with non-nil arguments

Move the known-non-nil scan outside the work loop to resolve an issue
with values that were declared outside the block being operated on.
Also consider phis whose arguments are all non-nil, as non-nil.

Change-Id: I4d5b840042de9eb181f2cb918f36913fb5d517a2
Reviewed-on: https://go-review.googlesource.com/13441
Reviewed-by: Keith Randall <khr@golang.org>