aarch64: Add some uses of force_lowpart_subreg

aarch64: Add some uses of force_lowpart_subreg

This patch makes more use of force_lowpart_subreg, similarly
to the recent patch for force_subreg.  The criteria were:

(1) The code is obviously specific to expand (where new pseudos
    can be created).

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

gcc/
	PR target/115464
	* config/aarch64/aarch64-builtins.cc (aarch64_expand_fcmla_builtin)
	(aarch64_expand_rwsr_builtin): Use force_lowpart_subreg instead of
	simplify_gen_subreg and lowpart_subreg.
	* config/aarch64/aarch64-sve-builtins-base.cc
	(svset_neonq_impl::expand): Likewise.
	* config/aarch64/aarch64-sve-builtins-sme.cc
	(add_load_store_slice_operand): Likewise.
	* config/aarch64/aarch64.cc (aarch64_sve_reinterpret): Likewise.
	(aarch64_addti_scratch_regs, aarch64_subvti_scratch_regs): Likewise.

gcc/testsuite/
	PR target/115464
	* gcc.target/aarch64/sve/acle/general/pr115464_2.c: New test.