cmd/5g: add missing splitclean.

See issue 887 for the 8g analogue.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/7306069

src/cmd/5g/ggen.c

@@ -609,6 +609,7 @@ cgen_shift(int op, int bounded, Node *nl, Node *nr, Node *res)
 		regalloc(&n3, types[TUINT32], N);
 		gmove(&lo, &n1);
 		gmove(&hi, &n3);
+		splitclean();
 		gins(ATST, &n3, N);
 		nodconst(&t, types[TUINT32], w);
 		p1 = gins(AMOVW, &t, &n1);

src/cmd/5g/gsubr.c

@@ -560,9 +560,9 @@ split64(Node *n, Node *lo, Node *hi)
 	if(!is64(n->type))
 		fatal("split64 %T", n->type);
 
-	sclean[nsclean].op = OEMPTY;
 	if(nsclean >= nelem(sclean))
 		fatal("split64 clean");
+	sclean[nsclean].op = OEMPTY;
 	nsclean++;
 	switch(n->op) {
 	default: