testsuite/97797 - adjust GIMPLE tests for sizetype

Tested on x86_64-unknown-linux-gnu, pushed.

2020-11-11  Richard Biener  <rguenther@suse.de>

	PR testsuite/97797
	* gcc.dg/torture/ssa-fre-5.c: Use __SIZETYPE__ where
	appropriate.
	* gcc.dg/torture/ssa-fre-6.c: Likewise.