libstdc++: Add missing constexpr to simd

libstdc++: Add missing constexpr to simd

The constexpr API is only available with -std=gnu++XX (and proposed for
C++26). The proposal is to have the complete simd API usable in constant
expressions.

This patch resolves several issues with using simd in constant
expressions.

Issues why constant_evaluated branches are necessary:
* subscripting vector builtins is not allowed in constant expressions
* if the implementation needs/uses memcpy
* if the implementation would otherwise call SIMD intrinsics/builtins

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
libstdc++-v3/ChangeLog:

	PR libstdc++/109261
	* include/experimental/bits/simd.h (_SimdWrapper::_M_set):
	Avoid vector builtin subscripting in constant expressions.
	(resizing_simd_cast): Avoid memcpy if constant_evaluated.
	(const_where_expression, where_expression, where)
	(__extract_part, simd_mask, _SimdIntOperators, simd): Add either
	_GLIBCXX_SIMD_CONSTEXPR (on public APIs), or constexpr (on
	internal APIs).
	* include/experimental/bits/simd_builtin.h (__vector_permute)
	(__vector_shuffle, __extract_part, _GnuTraits::_SimdCastType1)
	(_GnuTraits::_SimdCastType2, _SimdImplBuiltin)
	(_MaskImplBuiltin::_S_store): Add constexpr.
	(_CommonImplBuiltin::_S_store_bool_array)
	(_SimdImplBuiltin::_S_load, _SimdImplBuiltin::_S_store)
	(_SimdImplBuiltin::_S_reduce, _MaskImplBuiltin::_S_load): Add
	constant_evaluated case.
	* include/experimental/bits/simd_fixed_size.h
	(_S_masked_load): Reword comment.
	(__tuple_element_meta, __make_meta, _SimdTuple::_M_apply_r)
	(_SimdTuple::_M_subscript_read, _SimdTuple::_M_subscript_write)
	(__make_simd_tuple, __optimize_simd_tuple, __extract_part)
	(__autocvt_to_simd, _Fixed::__traits::_SimdBase)
	(_Fixed::__traits::_SimdCastType, _SimdImplFixedSize): Add
	constexpr.
	(_SimdTuple::operator[], _M_set): Add constexpr and add
	constant_evaluated case.
	(_MaskImplFixedSize::_S_load): Add constant_evaluated case.
	* include/experimental/bits/simd_scalar.h: Add constexpr.

	* include/experimental/bits/simd_x86.h (_CommonImplX86): Add
	constexpr and add constant_evaluated case.
	(_SimdImplX86::_S_equal_to, _S_not_equal_to, _S_less)
	(_S_less_equal): Value-initialize to satisfy constexpr
	evaluation.
	(_MaskImplX86::_S_load): Add constant_evaluated case.
	(_MaskImplX86::_S_store): Add constexpr and constant_evaluated
	case. Value-initialize local variables.
	(_MaskImplX86::_S_logical_and, _S_logical_or, _S_bit_not)
	(_S_bit_and, _S_bit_or, _S_bit_xor): Add constant_evaluated
	case.
	* testsuite/experimental/simd/pr109261_constexpr_simd.cc: New
	test.