PR rtl-optimization/106421: ICE in bypass_block from non-local goto.

PR rtl-optimization/106421: ICE in bypass_block from non-local goto.

This patch fixes PR rtl-optimization/106421, an ICE-on-valid (but
undefined) regression.  The fix, as proposed by Richard Biener, is to
defend against BLOCK_FOR_INSN returning NULL in cprop's bypass_block.

2023-01-10  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR rtl-optimization/106421
	* cprop.cc (bypass_block): Check that DEST is local to this
	function (non-NULL) before calling find_edge.

gcc/testsuite/ChangeLog
	PR rtl-optimization/106421
	* gcc.dg/pr106421.c: New test case.

(cherry picked from commit 851e1ba03f9de699a754dd8648fc151c3e26d697)