cmd/compile: fix Mul->Mul64 intrinsic alias

The alias declaration needs to come after the function it is aliasing.

It isn't a big deal in this case, as bits.Mul inlines and has as its
body bits.Mul64, so the desired code gets generated regardless.
The alias should only have an effect on inlining cost estimates
(for functions that call bits.Mul).

Change-Id: I0d814899ce7049a0fb36e8ce1ad5ababbaf6265f
Reviewed-on: https://go-review.googlesource.com/c/144597
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Giovanni Bajo <rasky@develer.com>

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

@@ -3474,12 +3474,12 @@ func init() {
 	addF("math/bits", "OnesCount",
 		makeOnesCountAMD64(ssa.OpPopCount64, ssa.OpPopCount32),
 		sys.AMD64)
-	alias("math/bits", "Mul", "math/bits", "Mul64", sys.ArchAMD64, sys.ArchARM64, sys.ArchPPC64)
 	addF("math/bits", "Mul64",
 		func(s *state, n *Node, args []*ssa.Value) *ssa.Value {
 			return s.newValue2(ssa.OpMul64uhilo, types.NewTuple(types.Types[TUINT64], types.Types[TUINT64]), args[0], args[1])
 		},
 		sys.AMD64, sys.ARM64, sys.PPC64)
+	alias("math/bits", "Mul", "math/bits", "Mul64", sys.ArchAMD64, sys.ArchARM64, sys.ArchPPC64)
 
 	addF("math/bits", "Add64",
 		func(s *state, n *Node, args []*ssa.Value) *ssa.Value {