Fix target/68124

[Richard Henderson <rth at redhat dot com>]

My fix for PR 67609 caused problems for -m32 -march=corei7,
a combination that I hadn't tested.

My feeling is that for post-reload splitters that want to
frob a register to a different mode, we ought to use something
more specific than gen_lowpart.  Perhaps adjust_reg_mode, or
perhaps something new and more specific.  But I'm not going to
try to audit the whole backend at this stage.

Anyway, I've tightened the test to the exact conditions that
caused 67609 problems in the expander; one can hope that everything
else is intentional in the backend and won't cause problems...

Committed.


r~

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