middle-end: Use subregs to expand COMPLEX_EXPR to set the lowpart.

middle-end: Use subregs to expand COMPLEX_EXPR to set the lowpart.

When lowering COMPLEX_EXPR we currently emit two VEC_EXTRACTs.  One for the
lowpart and one for the highpart.

The problem with this is that in RTL the lvalue of the RTX is the only thing
tying the two instructions together.

This means that e.g. combine is unable to try to combine the two instructions
for setting the lowpart and highpart.

For ISAs that have bit extract instructions we can eliminate one of the extracts
if, and only if we're setting the entire complex number.

This change changes the expand code when we're setting the entire complex number
to generate a subreg for the lowpart instead of a vec_extract.

This allows us to optimize sequences such as:

_Complex int f(int a, int b) {
    _Complex int t = a + b * 1i;
    return t;
}

from:

f:
	bfi     x2, x0, 0, 32
	bfi     x2, x1, 32, 32
	mov     x0, x2
	ret

into:

f:
	bfi	x0, x1, 32, 32
	ret

I have also confirmed the codegen for x86_64 did not change.

gcc/ChangeLog:

	* expmed.cc (store_bit_field_1): Add parameter that indicates if value is
	still undefined and if so emit a subreg move instead.
	(store_integral_bit_field): Likewise.
	(store_bit_field): Likewise.
	* expr.h (write_complex_part): Likewise.
	* expmed.h (store_bit_field): Add new parameter.
	* builtins.cc (expand_ifn_atomic_compare_exchange_into_call): Use new
	parameter.
	(expand_ifn_atomic_compare_exchange): Likewise.
	* calls.cc (store_unaligned_arguments_into_pseudos): Likewise.
	* emit-rtl.cc (validate_subreg): Likewise.
	* expr.cc (emit_group_store): Likewise.
	(copy_blkmode_from_reg): Likewise.
	(copy_blkmode_to_reg): Likewise.
	(clear_storage_hints): Likewise.
	(write_complex_part):  Likewise.
	(emit_move_complex_parts): Likewise.
	(expand_assignment): Likewise.
	(store_expr): Likewise.
	(store_field): Likewise.
	(expand_expr_real_2): Likewise.
	* ifcvt.cc (noce_emit_move_insn): Likewise.
	* internal-fn.cc (expand_arith_set_overflow): Likewise.
	(expand_arith_overflow_result_store): Likewise.
	(expand_addsub_overflow): Likewise.
	(expand_neg_overflow): Likewise.
	(expand_mul_overflow): Likewise.
	(expand_arith_overflow): Likewise.

gcc/testsuite/ChangeLog:

	* g++.target/aarch64/complex-init.C: New test.