[4/n][PR rtl-optimization/115877] Correct SUBREG handling in a destination

If we encounter something during SET handling that we can not handle, the safe
thing to do is to ignore the destination and continue the loop.

We've actually been trying to do slightly better with SUBREG destinations by
iterating into SUBREG_REG.  It turns out that wasn't working as expected.

The problem is once we "continue" we lose the state that we were inside the SET
and thus we ended up ignoring the destination completely rather than tracking
the SUBREG_REG object.  This could be fixed by restarting SET processing, but I
just don't see this as all that important to handle.  So rather than leave the
code as-is, not working per design, I'm twiddling it to use the common 'skip
subrtxs and continue' idiom used elsewhere.

This is a prerequisite for another patch in this series.  Specifically I have a
patch that explicitly tracks if we skipped a destination rather than trying to
imply it from the state of LIVE_TMP.  So this is probably NFC right now, but
that's a short-lived NFC.

Bootstrapped and regression tested on x86 and also run as part of a larger kit
on the crosses in my tester.

	PR rtl-optimization/115877
gcc/
	* ext-dce.cc (ext_dce_process_sets): More correctly handle SUBREG
	destinations.

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