Make tree-ssa-dse.c:normalize_ref return a bool

Make tree-ssa-dse.c:normalize_ref return a bool

This patch moves the check for an overlapping byte to normalize_ref
from its callers, so that it's easier to convert to poly_ints later.
It's not really worth it on its own.

2017-11-01  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-ssa-dse.c (normalize_ref): Check whether the ranges overlap
	and return false if not.
	(clear_bytes_written_by, live_bytes_read): Update accordingly.

From-SVN: r254313