PR middle-end/105853: Call store_constructor directly from calls.cc.

PR middle-end/105853: Call store_constructor directly from calls.cc.

This patch fixes both ICE regressions PR middle-end/105853 and
PR target/105856 caused by my recent patch to expand small const structs
as immediate constants.  That patch updated code generation in three
places: two in expr.cc that call store_constructor directly, and the
third in calls.cc's load_register_parameters that expands its CONSTRUCTOR
via expand_expr, as store_constructor is local/static to expr.cc, and
the "public" API, should usually simply forward the constructor to the
appropriate store_constructor function.

Alas, despite the clean regression testing on multiple targets, the above
ICEs show that expand_expr isn't a suitable proxy for store_constructor,
and things that (I'd assumed) shouldn't affect how/whether a struct is
placed in a register [such as whether the struct is considered packed/
aligned or not] actually interfere with the optimization that is being
attempted.

The (proposed) solution is to export store_constructor (and it's helper
function int_expr_size) from expr.cc, by removing their static qualifier
and prototyping both functions in expr.h, so they can be called directly
from load_register_parameters in calls.cc.  This cures both ICEs, but
almost as importantly improves code generation over GCC 12.

For PR 105853, GCC 12 generates:

compose_nd_na_ipv6_src:
	movzx eax, WORD PTR eth_addr_zero[rip+2]
	movzx edx, WORD PTR eth_addr_zero[rip]
	movzx edi, WORD PTR eth_addr_zero[rip+4]
	sal rax, 16
	or rax, rdx
	sal rdi, 32
	or rdi, rax
	xor eax, eax
	jmp packet_set_nd
eth_addr_zero:	.zero 6

where now (with this fix) GCC 13 generates:
compose_nd_na_ipv6_src:
        xorl    %edi, %edi
        xorl    %eax, %eax
        jmp     packet_set_nd

Likewise, for PR 105856 on ARM, we'd previously generate:
g_329_3:
	movw r3, #:lower16:.LANCHOR0
	movt r3, #:upper16:.LANCHOR0
	ldr r0, [r3]
	b func_19

but with this optimization we now generate:
g_329_3:
        mov     r0, #6
        b       func_19

2022-06-07  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR middle-end/105853
	PR target/105856
	* calls.cc (load_register_parameters): Call store_constructor
	and int_expr_size directly instead of expanding via expand_expr.
	* expr.cc (static void store_constructor): Don't prototype here.
	(static HOST_WIDE_INT int_expr_size): Likewise.
	(store_constructor): No longer static.
	(int_expr_size): Likewise, no longer static.
	* expr.h (store_constructor): Prototype here.
	(int_expr_size): Prototype here.

gcc/testsuite/ChangeLog
	PR middle-end/105853
	PR target/105856
	* gcc.dg/pr105853.c: New test case.
	* gcc.dg/pr105856.c: New test case.