Add force_lowpart_subreg

Add force_lowpart_subreg

optabs had a local function called lowpart_subreg_maybe_copy
that is very similar to the lowpart version of force_subreg.
This patch adds a force_lowpart_subreg wrapper around
force_subreg and uses it in optabs.cc.

The only difference between the old and new functions is that
the old one asserted success while the new one doesn't.
It's common not to assert elsewhere when taking subregs;
normally a null result is enough.

Later patches will make more use of the new function.

gcc/
	* explow.h (force_lowpart_subreg): Declare.
	* explow.cc (force_lowpart_subreg): New function.
	* optabs.cc (lowpart_subreg_maybe_copy): Delete.
	(expand_absneg_bit): Use force_lowpart_subreg instead of
	lowpart_subreg_maybe_copy.
	(expand_copysign_bit): Likewise.