[dev.ssa] cmd/compile: minor cleanup

Cull dead code. Add TODOs.

Change-Id: I81d24371de47f5a27d3a3a0ec0ef5baaf6814c06
Reviewed-on: https://go-review.googlesource.com/12659
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/cmd/compile/internal/ssa/gen/genericOps.go b/src/cmd/compile/internal/ssa/gen/genericOps.go
index 6129849..4014fd5 100644
--- a/src/cmd/compile/internal/ssa/gen/genericOps.go
+++ b/src/cmd/compile/internal/ssa/gen/genericOps.go
@@ -153,7 +153,7 @@
 	{name: "Convert"}, // convert arg0 to another type
 	{name: "ConvNop"}, // interpret arg0 as another type
 
-	// Safety checks
+	// Automatically inserted safety checks
 	{name: "IsNonNil"},   // arg0 != nil
 	{name: "IsInBounds"}, // 0 <= arg0 < arg1