cmd/8g: don't allocate a register early for cap(CHAN).

There is no reason to generate different code for cap and len.

Fixes #8025.
Fixes #8026.

LGTM=rsc
R=rsc, iant, khr
CC=golang-codereviews
https://golang.org/cl/93570044

src/cmd/8g/cgen.c

@@ -347,8 +347,11 @@ cgen(Node *n, Node *res)
 		if(istype(nl->type, TCHAN)) {
 			// chan has cap in the second 32-bit word.
 			// a zero pointer means zero length
-			regalloc(&n1, types[tptr], res);
+			tempname(&n1, types[tptr]);
 			cgen(nl, &n1);
+			regalloc(&n2, types[tptr], N);
+			gmove(&n1, &n2);
+			n1 = n2;
 
 			nodconst(&n2, types[tptr], 0);
 			gins(optoas(OCMP, types[tptr]), &n1, &n2);

test/torture.go

@@ -337,3 +337,10 @@ func ChainDivConst(a int) int {
 func ChainMulBytes(a, b, c byte) byte {
 	return a*(a*(a*(a*(a*(a*(a*(a*(a*b+c)+c)+c)+c)+c)+c)+c)+c) + c
 }
+
+func ChainCap() {
+	select {
+	case <-make(chan int, cap(make(chan int, cap(make(chan int, cap(make(chan int, cap(make(chan int))))))))):
+	default:
+	}
+}