Fix HWI + -unsigned in combine.c

rtx_equal_for_field_assignment_p had:

 	x = adjust_address_nv (x, GET_MODE (y),
			       -subreg_lowpart_offset (GET_MODE (x),
						       GET_MODE (y)));

But subreg_lowpart_offset returns an unsigned int and
adjust_address_nv takes a HWI, so a subreg offset of 4 would
give a memory offset of 0x00000000fffffffffc.

2017-10-23  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* combine.c (rtx_equal_for_field_assignment_p): Use
	byte_lowpart_offset.

From-SVN: r253997

gcc/ChangeLog

+2017-10-23  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	* combine.c (rtx_equal_for_field_assignment_p): Use
+	byte_lowpart_offset.
+
 2017-10-22  Richard Sandiford  <richard.sandiford@linaro.org>
 	    Alan Hayward  <alan.hayward@arm.com>
 	    David Sherwood  <david.sherwood@arm.com>

gcc/combine.c

@@ -9526,13 +9526,9 @@ rtx_equal_for_field_assignment_p (rtx x, rtx y, bool widen_x)
 	return 0;
       if (BYTES_BIG_ENDIAN != WORDS_BIG_ENDIAN)
 	return 0;
-      /* For big endian, adjust the memory offset.  */
-      if (BYTES_BIG_ENDIAN)
-	x = adjust_address_nv (x, GET_MODE (y),
-			       -subreg_lowpart_offset (GET_MODE (x),
-						       GET_MODE (y)));
-      else
-	x = adjust_address_nv (x, GET_MODE (y), 0);
+      x = adjust_address_nv (x, GET_MODE (y),
+			     byte_lowpart_offset (GET_MODE (y),
+						  GET_MODE (x)));
     }
 
   if (x == y || rtx_equal_p (x, y))