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cc2/qbe: Change lhs() to return Node

Receiving a pointer to the struct were we want to store the
result needs a lot of temporary variables in the code. If
we make that lhs() returns a struct instead of a pointer to
struct we can remove a lot of these temporaries and
simplify the code. The code generated is pretty similar
because returning structs usually mean passing an additional
pointer to the function lhsed where it can write the results.

--- a/src/cmd/cc/cc2/target/qbe/cgen.c

+++ b/src/cmd/cc/cc2/target/qbe/cgen.c

@@ -338,19 +338,17 @@

 }

 static Node *

-lhs(Node *np, Node *new)

+lhs(Node *np)

 {

 	switch (np->op) {

 	case OREG:

 	case OMEM:

 	case OAUTO:

-		*new = *np;

-		return new;

+		return np;

 	case OPTR:

-		return rhs(np->left, new);

+		return rhs(np->left, node(OTMP));

 	case OFIELD:

-		*new = *field(np, 1);

-		return new;

+		return field(np, 1);

 	default:

 		abort();

 	}

@@ -595,7 +593,7 @@

 			aux1.right = r;

 			aux1.type = np->type;

 			rhs(&aux1, &aux2);

-			lhs(l, &aux1);

+			aux1 = *lhs(l);

 			assign(tp, &aux1, &aux2);

 			break;

 		default:

@@ -606,11 +604,11 @@

 			r = rhs(&aux2, &aux1);

 		case 0:

 			if (l->complex >= r->complex) {

-				lhs(l, &aux2);

+				aux2 = *lhs(l);

 				rhs(r, ret);

 			} else {

 				rhs(r, ret);

-				lhs(l, &aux2);

+				aux2 = *lhs(l);

 			}

 			return assign(tp, &aux2, ret);

@@ -625,7 +623,7 @@

 		*ret = *load(tp, rhs(l, &aux1));

 		return ret;

 	case OADDR:

-		lhs(l, ret);

+		*ret = *lhs(l);

 		ret->type = *tp;

 		return ret;

 	case OFIELD: