cmd/compile: disable checknils during alg eq generation

Cuts 20k off cmd/go and 32k off golang.org/x/tools/cmd/godoc, approx 0.15% each.

For #6853 and #9930

Change-Id: Ic510b76b80a9153b1ede7b3533d2dbc16caa5c63
Reviewed-on: https://go-review.googlesource.com/19768
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/compile/internal/gc/subr.go

@@ -2862,10 +2862,18 @@ func geneq(sym *Sym, t *Type) {
 	// for a struct containing a reflect.Value, which itself has
 	// an unexported field of type unsafe.Pointer.
 	old_safemode := safemode
-
 	safemode = 0
+
+	// Disable checknils while compiling this code.
+	// We are comparing a struct or an array,
+	// neither of which can be nil, and our comparisons
+	// are shallow.
+	Disable_checknil++
+
 	funccompile(fn)
+
 	safemode = old_safemode
+	Disable_checknil--
 }
 
 func ifacelookdot(s *Sym, t *Type, followptr *bool, ignorecase int) *Type {