Fix pessimistic DImode handling in combine.c:make_field_assignment

The make_field_assignment code:

      src = force_to_mode (src, mode,
		       GET_MODE_PRECISION (mode) >= HOST_BITS_PER_WIDE_INT
		       ? HOST_WIDE_INT_M1U
		       : (HOST_WIDE_INT_1U << len) - 1,
		       0);

would ignore the field length len for DImode, even though DImode can be
handled using HWIs.  I think the code should be testing len instead.

2017-06-12  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* combine.c (make_field_assignment): Check len rather than the mode
	precision when calling force_to_mode.

From-SVN: r249128

gcc/ChangeLog

@@ -1,3 +1,8 @@
+2017-06-12  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	* combine.c (make_field_assignment): Check len rather than the mode
+	precision when calling force_to_mode.
+
 2017-06-12  Georg-Johann Lay  <avr@gjlay.de>
 
 	Support multilibs and devices that see flash in RAM address range.

gcc/combine.c

@@ -9634,7 +9634,7 @@ make_field_assignment (rtx x)
 						     other, pos),
 			       dest);
   src = force_to_mode (src, mode,
-		       GET_MODE_PRECISION (mode) >= HOST_BITS_PER_WIDE_INT
+		       len >= HOST_BITS_PER_WIDE_INT
 		       ? HOST_WIDE_INT_M1U
 		       : (HOST_WIDE_INT_1U << len) - 1,
 		       0);