cmd/compile: disable use of new saturating float-to-int conversions

The new conversions can be activated (or bisected) with
  -gcflags=all=-d=converthash=PATTERN

where PATTERN is either a hash string or n, qn, y, qy for
no, quietly no, yes, quietly yes.

This CL makes the default pattern be "qn" instead of the
default-default which is an efficient encoding of "qy".

Updates #75834

Change-Id: I88a9fd7880bc999132420c8d0a22a8fdc1e95a2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/711845
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Bypass: David Chase <drchase@google.com>

src/cmd/compile/internal/base/flag.go

@@ -264,6 +264,9 @@ func ParseFlags() {
 
 	if Debug.Converthash != "" {
 		ConvertHash = NewHashDebug("converthash", Debug.Converthash, nil)
+	} else {
+		// quietly disable the convert hash changes
+		ConvertHash = NewHashDebug("converthash", "qn", nil)
 	}
 	if Debug.Fmahash != "" {
 		FmaHash = NewHashDebug("fmahash", Debug.Fmahash, nil)

test/codegen/math.go

@@ -1,4 +1,4 @@
-// asmcheck
+// asmcheck -gcflags=-d=converthash=qy
 
 // Copyright 2018 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style

test/convert5.go

@@ -1,4 +1,4 @@
-// run
+// run -gcflags=-d=converthash=qy
 
 // Copyright 2020 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style