test: port bits.Len intrinsics tests to the new codegen harness

This change move bits.Len* intrinsification tests to the new codegen
test harness, removing them from the old ssa_test file. Five different
test functions (one for each bit.Len function tested) was used, to
avoid possible unwanted interactions between multiple calls inside one
function.

Change-Id: Iffd5be55b58e88597fa30a562a28dacb01236d8b
Reviewed-on: https://go-review.googlesource.com/98156
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Giovanni Bajo <rasky@develer.com>

test/codegen/mathbits.go

@@ -0,0 +1,53 @@
+// asmcheck
+
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package codegen
+
+import "math/bits"
+
+// --------------- //
+//    bits.Len*    //
+// --------------- //
+
+func Len(n uint) int {
+	//amd64:"BSRQ"
+	//s390x:"FLOGR"
+	//arm:"CLZ" arm64:"CLZ"
+	//mips:"CLZ"
+	return bits.Len(n)
+}
+
+func Len64(n uint64) int {
+	//amd64:"BSRQ"
+	//s390x:"FLOGR"
+	//arm:"CLZ" arm64:"CLZ"
+	//mips:"CLZ"
+	return bits.Len64(n)
+}
+
+func Len32(n uint32) int {
+	//amd64:"BSRQ"
+	//s390x:"FLOGR"
+	//arm:"CLZ" arm64:"CLZ"
+	//mips:"CLZ"
+	return bits.Len32(n)
+}
+
+func Len16(n uint16) int {
+	//amd64:"BSRQ"
+	//s390x:"FLOGR"
+	//arm:"CLZ" arm64:"CLZ"
+	//mips:"CLZ"
+	return bits.Len16(n)
+}
+
+func Len8(n uint8) int {
+	//amd64 Len8 not intrisified
+	//s390x:"FLOGR"
+	//arm:"CLZ" arm64:"CLZ"
+	//mips:"CLZ"
+	return bits.Len8(n)
+}