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cc: fix wrong "useless or misleading comparison" warning

to reproduce:

 	u8int x, y;

 	x = 0xff;
 	y = 0xc0;
 	if((s8int)(x & y) >= 0)
 		print("help\n");

compiles correctly but prints a warning

warning: test.c:11 useless or misleading comparison: UINT >= 0

the issue is that compar() unconditionally skipped over
all left casts ignoring the case when a cast would sign
extend the value.

the new code only skips over the cast when the original
type with is smaller than the cast result or when they
are equal width and types have same signedness. so the
effective left hand side type is the last truncation
or sign extension.

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

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

@@ -1373,15 +1373,22 @@

 	/*

 	 * Skip over left casts to find out the original expression range.

 	 */

-	while(l->op == OCAST)

+	while(l->op == OCAST){

+		lt = l->type;

+		rt = l->left->type;

+		if(lt == T || rt == T)

+			return 0;

+		if(lt->width < rt->width)

+			break;

+		if(lt->width == rt->width && ((lt->etype ^ rt->etype) & 1) != 0)

+			break;

 		l = l->left;

+	}

 	if(l->op == OCONST)

 		return 0;

 	lt = l->type;

-	if(lt == T)

+	if(lt == T || lt->etype == TXXX || lt->etype > TUVLONG)

 		return 0;

-	if(lt->etype == TXXX || lt->etype > TUVLONG)

-		return 0;

 	/*

 	 * Skip over the right casts to find the on-screen value.

@@ -1399,16 +1406,17 @@

 	if((rt->etype&1) && r->vconst < 0)	/* signed negative */

 		x.a = ~0ULL;

-	if((lt->etype&1)==0){

+	if(lt->etype & 1){

+		/* signed */

+		lo = big(~0ULL, -(1LL<<(lt->width*8-1)));

+		hi = big(0, (1LL<<(lt->width*8-1))-1);

+	} else {

 		/* unsigned */

 		lo = big(0, 0);

 		if(lt->width == 8)

 			hi = big(0, ~0ULL);

 		else

-			hi = big(0, (1LL<<(l->type->width*8))-1);

-	}else{

-		lo = big(~0ULL, -(1LL<<(l->type->width*8-1)));

-		hi = big(0, (1LL<<(l->type->width*8-1))-1);

+			hi = big(0, (1LL<<(lt->width*8))-1);

 	}

 	switch(op){

@@ -1462,4 +1470,3 @@

 	warn(n, "useless or misleading comparison: %s", cmpbuf);

 	return 0;

 }

-