expand: Fold x - y < 0 to x < y during expansion [PR94802]

expand: Fold x - y < 0 to x < y during expansion [PR94802]

My earlier patch to simplify x - y < 0 etc. for signed subtraction
with undefined overflow into x < y in match.pd regressed some tests,
even when it was guarded to be post-IPA, the following patch thus
attempts to optimize that during expansion instead (which is the last
time we can do it, afterwards we lose the information whether it was
x - y < 0 or (int) ((unsigned) x - y) < 0 for which we couldn't
optimize it.

2021-01-05  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/94802
	* expr.h (maybe_optimize_sub_cmp_0): Declare.
	* expr.c: Include tree-pretty-print.h and flags.h.
	(maybe_optimize_sub_cmp_0): New function.
	(do_store_flag): Use it.
	* cfgexpand.c (expand_gimple_cond): Likewise.

	* gcc.target/i386/pr94802.c: New test.
	* gcc.dg/Wstrict-overflow-25.c: Remove xfail.