Handle CONST_POLY_INTs in CONST_VECTORs [PR97141, PR98726]

This PR is caused by POLY_INT_CSTs being (necessarily) valid
in tree-level VECTOR_CSTs but CONST_POLY_INTs not being valid
in RTL CONST_VECTORs.  I can't tell/remember how deliberate
that was, but I'm guessing not very.  In particular,
valid_for_const_vector_p was added to guard against symbolic
constants rather than CONST_POLY_INTs.

I did briefly consider whether we should maintain the current
status anyway.  However, that would then require a way of
constructing variable-length vectors from individiual elements
if, say, we have:

   { [2, 2], [3, 2], [4, 2], … }

So I'm chalking this up to an oversight.  I think the intention
(and certainly the natural thing) is to have the same rules for
both trees and RTL.

The SVE CONST_VECTOR code should already be set up to handle
CONST_POLY_INTs.  However, we need to add support for Advanced SIMD
CONST_VECTORs that happen to contain SVE-based values.  The patch does
that by expanding such CONST_VECTORs in the same way as variable vectors.

gcc/
	PR rtl-optimization/97141
	PR rtl-optimization/98726
	* emit-rtl.c (valid_for_const_vector_p): Return true for
	CONST_POLY_INT_P.
	* rtx-vector-builder.h (rtx_vector_builder::step): Return a
	poly_wide_int instead of a wide_int.
	(rtx_vector_builder::apply_set): Take a poly_wide_int instead
	of a wide_int.
	* rtx-vector-builder.c (rtx_vector_builder::apply_set): Likewise.
	* config/aarch64/aarch64.c (aarch64_legitimate_constant_p): Return
	false for CONST_VECTORs that cannot be forced to memory.
	* config/aarch64/aarch64-simd.md (mov<mode>): If a CONST_VECTOR
	is too complex to force to memory, build it up from individual
	elements instead.

gcc/testsuite/
	PR rtl-optimization/97141
	PR rtl-optimization/98726
	* gcc.c-torture/compile/pr97141.c: New test.
	* gcc.c-torture/compile/pr98726.c: Likewise.
	* gcc.target/aarch64/sve/pr97141.c: Likewise.
	* gcc.target/aarch64/sve/pr98726.c: Likewise.

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