ifcvt: Fix -fcompare-debug bug [PR100852]

ifcvt: Fix -fcompare-debug bug [PR100852]

The following testcase fails -fcompare-debug, because it is ifcvt optimized
into umin only with -g0 and not with -g - the function(s) use
prev_nonnote_insn, which without -g finds a real insn the code is looking
for, while with -g finds a DEBUG_INSN.

2021-06-10  Jakub Jelinek  <jakub@redhat.com>

	PR debug/100852
	* ifcvt.c (noce_get_alt_condition, noce_try_abs): Use
	prev_nonnote_nondebug_insn instead of prev_nonnote_insn.

	* g++.dg/opt/pr100852.C: New test.