[PR87815]Don't generate shift sequence for load replacement in DSE when the mode size is not compile-time constant

The patch adds a check if the gap is compile-time constant.

This happens when dse decides to replace the load with previous store value.
The problem is that, shift sequence could not accept compile-time non-constant
mode operand.

gcc/

2018-11-12  Renlin Li  <renlin.li@arm.com>

	PR target/87815
	* dse.c (get_stored_val): Add check for compile-time
	constantness of gap.

gcc/testsuite/

2018-11-12  Renlin Li  <renlin.li@arm.com>

	PR target/87815
	* gcc.target/aarch64/sve/pr87815.c: New.

From-SVN: r266033