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.

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32},
with no new failures.  Ok for mainline?


2023-01-09  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.


Thanks in advance,
Roger
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