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

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