PR rtl-optimization/112380: Defend against CLOBBERs in combine.cc

This patch addresses PR rtl-optimization/112380, an ICE-on-valid regression
where a (clobber (const_int 0)) encounters a sanity checking gcc_assert
(at line 7554) in simplify-rtx.cc.  These CLOBBERs are used internally
by GCC's combine pass much like error_mark_node is used by various
language front-ends.

The solutions are either to handle/accept these CLOBBERs through-out
(or in more places in) the middle-end's RTL optimizers, including functions
in simplify-rtx.cc that are used by passes other than combine, and/or
attempt to prevent these CLOBBERs escaping from try_combine into the
RTX/RTL stream.  The benefit of the second approach is that it actually
allows for better optimization: when try_combine fails to simplify an
expression instead of substituting a CLOBBER to avoid the instruction
pattern being recognized, noticing the CLOBBER often allows combine
to attempt alternate simplifications/transformations looking for those
that can be recognized.

This first alternative is the minimal fix to address the CLOBBER
encountered in the bugzilla PR.

2023-12-11  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR rtl-optimization/112380
	* combine.cc (expand_field_assignment): Check if gen_lowpart
	returned a CLOBBER, and avoid calling gen_simplify_binary with
	it if so.

gcc/testsuite/ChangeLog
	PR rtl-optimization/112380
	* gcc.dg/pr112380.c: New test case.