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cc2/qbe: Use sethi number to drive assign generation

Sethi numbers gives an indication of the number of registers
needed to implement a tree, and generating first the subtree
with an higher sethi number improves the performance of  the
generated code.

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

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

@@ -585,9 +585,14 @@

 				l = l->left;

 			}

 		case 0:

-			/* TODO: see what is the most difficult */

-			lhs(l, &aux2);

-			rhs(r, ret);

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

+				lhs(l, &aux2);

+				rhs(r, ret);

+			} else {

+				rhs(r, ret);

+				lhs(l, &aux2);

+			}

+

 			return assign(tp, &aux2, ret);

 		}

 		return ret;