Revert one of the force_subreg changes

Revert one of the force_subreg changes

One of the changes in g:d4047da6a070175aae7121c739d1cad6b08ff4b2
caused a regression in ft32-elf; see:

    https://gcc.gnu.org/pipermail/gcc-patches/2024-June/655418.html

for details.  This change was different from the others in that the
original call was to simplify_subreg rather than simplify_lowpart_subreg.
The old code would therefore go on to do the force_reg for more cases
than the new code would.

gcc/
	* expmed.cc (store_bit_field_using_insv): Revert earlier change
	to use force_subreg instead of simplify_gen_subreg.