[dev.ssa] cmd/compile: rewrite user nil check as OpIsNonNil

[dev.ssa] cmd/compile: rewrite user nil check as OpIsNonNil

Rewite user nil checks as OpIsNonNil so our nil check elimination pass
can take advantage and remove redundant checks.

With make.bash this removes 10% more nilchecks (34110 vs 31088).

Change-Id: Ifb01d1b6d2d759f5e2a5aaa0470e1d5a2a680212
Reviewed-on: https://go-review.googlesource.com/14321
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>