Do the usual dance when assigning to SUBREG_PROMOTED_VAR_P destinations:
first convert to the outer mode, then extend to the inner mode.

Tested that it fixes the powerpc64le-linux-gnu breakage.  Also tested
on x86_64-linux-gnu and powerpc64-linux-gnu.

gcc/
	PR bootstrap/68393
	* internal-fn.c (expand_direct_optab_fn): Handle SUBREG_PROMOTED_VAR_P
	destinations.

From-SVN: r230590