Fix mode_for_size units in caller-save.c (PR83761)

The new opt_mode asserts triggered for replace_reg_with_saved_mem
because it was passing bytes rather than bits to mode_for_size.
Previously we ended up with a BLKmode register instead, but
presumably that didn't matter because this is "only" used for
debug insns.

2018-01-11  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR rtl-optimization/83761
	* caller-save.c (replace_reg_with_saved_mem): Pass bits rather
	than bytes to mode_for_size.

From-SVN: r256525

gcc/ChangeLog

+2018-01-11  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	PR rtl-optimization/83761
+	* caller-save.c (replace_reg_with_saved_mem): Pass bits rather
+	than bytes to mode_for_size.
+
 2018-01-10  Jan Hubicka  <hubicka@ucw.cz>
 
 	PR middle-end/83189

gcc/caller-save.c

@@ -1147,7 +1147,8 @@ replace_reg_with_saved_mem (rtx *loc,
 	    machine_mode smode = save_mode[regno];
 	    gcc_assert (smode != VOIDmode);
 	    if (hard_regno_nregs (regno, smode) > 1)
-	      smode = mode_for_size (exact_div (GET_MODE_SIZE (mode), nregs),
+	      smode = mode_for_size (exact_div (GET_MODE_BITSIZE (mode),
+						nregs),
 				     GET_MODE_CLASS (mode), 0).require ();
 	    XVECEXP (mem, 0, i) = gen_rtx_REG (smode, regno + i);
 	  }