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.

gcc/dse.c

@@ -1757,8 +1757,7 @@ find_shift_sequence (poly_int64 access_size,
      the machine.  */
 
   opt_scalar_int_mode new_mode_iter;
-  FOR_EACH_MODE_FROM (new_mode_iter,
-		      smallest_int_mode_for_size (GET_MODE_BITSIZE (read_mode)))
+  FOR_EACH_MODE_IN_CLASS (new_mode_iter, MODE_INT)
     {
       rtx target, new_reg, new_lhs;
       rtx_insn *shift_seq, *insn;
@@ -1767,6 +1766,8 @@ find_shift_sequence (poly_int64 access_size,
       new_mode = new_mode_iter.require ();
       if (GET_MODE_BITSIZE (new_mode) > BITS_PER_WORD)
 	break;
+      if (maybe_lt (GET_MODE_SIZE (new_mode), GET_MODE_SIZE (read_mode)))
+	continue;
 
       /* Try a wider mode if truncating the store mode to NEW_MODE
 	 requires a real instruction.  */

gcc/testsuite/gcc.target/aarch64/sve/acle/general/pr98037.c

@@ -0,0 +1,6 @@
+/* { dg-options "-msve-vector-bits=1024 -O3" } */
+
+typedef __SVInt8_t vec __attribute__((arm_sve_vector_bits(1024)));
+struct pair { vec v[2]; };
+void use (struct pair *);
+vec f (struct pair p) { vec v = p.v[1]; use (&p); return v; }