runtime: please vet

The current code is correct, but vet does not understand it:
asm_amd64.s:963: [amd64] invalid MOVL of ret+0(FP); int64 is 8-byte value
asm_amd64.s:964: [amd64] invalid offset ret+4(FP); expected ret+0(FP)

LGTM=minux
R=golang-codereviews, minux
CC=golang-codereviews
https://golang.org/cl/125200044

src/pkg/runtime/asm_amd64.s

@@ -955,8 +955,9 @@ TEXT runtime·cputicks(SB),NOSPLIT,$0-0
 
 TEXT runtime·gocputicks(SB),NOSPLIT,$0-8
 	RDTSC
-	MOVL    AX, ret+0(FP)
-	MOVL    DX, ret+4(FP)
+	SHLQ    $32, DX
+	ADDQ    DX, AX
+	MOVQ    AX, ret+0(FP)
 	RET
 
 TEXT runtime·stackguard(SB),NOSPLIT,$0-16