dse: Cope with bigger-than-integer modes [PR98037]

dse: Cope with bigger-than-integer modes [PR98037]

dse.c:find_shift_sequence tries to represent a store and load
back as a shift right followed by a truncation.  It therefore
needs to find an integer mode in which to do the shift right.
The loop it uses has the form:

  FOR_EACH_MODE_FROM (new_mode_iter,
		      smallest_int_mode_for_size (GET_MODE_BITSIZE (read_mode)))

which implicitly assumes that read_mode has an equivalent integer mode.
As shown in the testcase, not all modes have such an integer mode.

This patch just makes the code start from the smallest integer mode and
skip modes that are too small.  The loop already breaks at the first
mode wider than word_mode.

gcc/
	PR rtl-optimization/98037
	* dse.c (find_shift_sequence): Iterate over all integers and
	skip modes that are too small.

gcc/testsuite/
	PR rtl-optimization/98037
	* gcc.target/aarch64/sve/acle/general/pr98037.c: New test.