ir: Cleanup and correct comments of smaller_mode().

include/libfirm/irmode.h

@@ -291,7 +291,7 @@ FIRM_API int mode_is_data (const ir_mode *mode);
  * Returns true if a value of mode @p sm can be converted to mode @p lm without
  * loss.
  *
- * That is the interpretation of the numbers does not changes, so you a signed
+ * That is the interpretation of the numbers does not changes, so a signed
  * integer mode is never smaller than an unsigned integer mode since the
  * unsigned mode can't represent negative numbers in a way that they are
  * interpreted as negative numbers.
@@ -304,7 +304,7 @@ FIRM_API int smaller_mode(const ir_mode *sm, const ir_mode *lm);
  * Returns true if no information is lost when converting a value of mode @p sm
  * into mode @p lm (and back to mode @p sm).
  *
- * So the interpretation of the values may change in the intermediate mode @p sm
+ * So the interpretation of the values may change in the intermediate mode @p lm
  * (for example when converting negative signed integer numbers into unsigned
  * integers) but after a conversion back they are exactly the same value.
  *

ir/ir/irmode.c

@@ -356,24 +356,18 @@ int smaller_mode(const ir_mode *sm, const ir_mode *lm)
 			assert(get_mode_arithmetic(sm) == irma_twos_complement);
 
 			/* integers are convertable if
-			 *   - both have the same sign and lm is the larger one
-			 *   - lm is the signed one and is at least two bits larger
-			 *     (one for the sign, one for the highest bit of sm)
-			 *   - sm & lm are two_complement and lm has greater or equal
-			 *     number of bits
-			 */
+			 * - both have the same sign and lm is the larger one
+			 * - lm is signed and is at least one bit larger (the sign) */
 			unsigned sm_bits = get_mode_size_bits(sm);
 			unsigned lm_bits = get_mode_size_bits(lm);
 			if (mode_is_signed(sm)) {
 				if (!mode_is_signed(lm))
-					return 0;
-				return sm_bits <= lm_bits;
+					return false;
 			} else {
-				if (mode_is_signed(lm)) {
+				if (mode_is_signed(lm))
 					return sm_bits < lm_bits;
-				}
-				return sm_bits <= lm_bits;
 			}
+			return sm_bits <= lm_bits;
 
 		case irms_auxiliary:
 		case irms_data: