Make more use of force_subreg

Make more use of force_subreg

This patch makes target-independent code use force_subreg instead
of simplify_gen_subreg in some places.  The criteria were:

(1) The code is obviously specific to expand (where new pseudos
    can be created), or at least would be invalid to call when
    !can_create_pseudo_p () and temporaries are needed.

(2) The value is obviously an rvalue rather than an lvalue.

(3) The offset wasn't a simple lowpart or highpart calculation;
    a later patch will deal with those.

Doing this should reduce the likelihood of bugs like PR115464
occuring in other situations.

gcc/
	* expmed.cc (store_bit_field_using_insv): Use force_subreg
	instead of simplify_gen_subreg.
	(store_bit_field_1): Likewise.
	(extract_bit_field_as_subreg): Likewise.
	(extract_integral_bit_field): Likewise.
	(emit_store_flag_1): Likewise.
	* expr.cc (convert_move): Likewise.
	(convert_modes): Likewise.
	(emit_group_load_1): Likewise.
	(emit_group_store): Likewise.
	(expand_assignment): Likewise.