test: fix nilptr5 for AIX

This commit fixes a mistake made in CL 144538.
This nilcheck can be removed because OpPPC64LoweredMove will fault if
arg0 is nil, as it's used to store. Further information can be found in
cmd/compile/internal/ssa/nilcheck.go.

Change-Id: Ifec0080c00eb1f94a8c02f8bf60b93308e71b119
Reviewed-on: https://go-review.googlesource.com/c/151298
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

test/nilptr5_aix.go

@@ -23,7 +23,7 @@ type T [29]byte
 
 func f6(p, q *T) {
 	x := *p // ERROR "generated nil check"
-	*q = x  // ERROR "generated nil check"
+	*q = x  // ERROR "removed nil check"
 }
 
 // make sure to remove nil check for memory move (issue #18003)