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cc: don't rearrange constant scaling across unsigned casts

On 64-bit platforms, when the index being 32-bit unsigned,
any calculation there must be done modulo 2^32,
but rearranging the factors into the address calculation
is done in signed 64-bits yielding the wrong index.

--- a/sys/src/cmd/cc/scon.c

+++ b/sys/src/cmd/cc/scon.c

@@ -587,8 +587,16 @@

 	switch(n->op) {

 	case OCAST:

-		if(nilcast(n->left->type, n->type))

-			return 1;

+		if(nilcast(n->left->type, n->type)){

+			/*

+			 * must not cross casts from unsigned

+			 * when the modulus changes to handle

+			 * overflow correctly.

+			 */

+			if(!typeu[n->left->type->etype]

+			|| ewidth[n->left->type->etype] == ewidth[n->type->etype])

+				return 1;

+		}

 		break;

 	case ONEG: