Restrict gcc.dg/tree-ssa/ssa-dse-26.c

This restricts the testcase to the target where it exposes the
situation fixed with g:e9d297a1
which targets BIT_FIELD_REFs created by fold_truth_andor.

This avoids strange dejagnu limits with overly long target lists
and simplifies the tests.

2021-05-04  Richard Biener  <rguenther@suse.de>

	* gcc.dg/tree-ssa/ssa-dse-26.c: Skip on !lp64 targets,
	simplify dump scanning down to one case.