fix latent bootstrap-debug issue

fix latent bootstrap-debug issue

I've hit a bootstrap-debug error involving large subprograms in
gcc/ada/sem_ch12.adb.  I'm afraid I couldn't narrow it down to a
reasonable testcase.

thread1 made different decisions about a block containing a
builtin_eh_filter call because in one compilation, estimate_num_insns
found a cgraph_node for the builtin and could thus get to the
is_simple_builtin test, but in the other it didn't.  With different
insn counts, one stage jump-threaded and the other didn't, and the
resulting code diverged quite a bit.

The reason the builtin had a cgraph_node in one case but not the other
was that modref got a chance to analyze the builtin call when it was
the first stmt in the block, and that created the cgraph_node.
However, when it was preceded by debug stmts, the loop in
analyze_function was cut short after the first debug stmt, because the
summary so far was not useful.

This patch fixes both issues: skip debug stmts in the analyze_function
loop, so as to prevent them from affecting any decisions in the loop,
and enable the insn count estimator to get to the is_simple_builtin
test when a cgraph_node has not been created for the builtin.

for  gcc/ChangeLog

	* ipa-modref.c (analyze_function): Skip debug stmts.
	* tree-inline.c (estimate_num_insn): Consider builtins even
	without a cgraph_node.