bitint: Fix up stores to large/huge _BitInt bitfields [PR114329]

The verifier requires BIT_FIELD_REFs with INTEGRAL_TYPE_P first operand
to have mode precision.  In most cases for the large/huge _BitInt bitfield
stores the code uses bitfield representatives, which are typically arrays
of chars, but if the bitfield starts at byte boundary on big endian,
the code uses as nlhs in lower_mergeable_store COMPONENT_REF of the
bitfield FIELD_DECL instead, which is fine for the limb accesses,
but when used for the most significant limb can result in invalid
BIT_FIELD_REF because the first operand then has BITINT_TYPE and
usually VOIDmode.

The following patch adds a helper method for the 4 creatikons of
BIT_FIELD_REF which when needed adds a VIEW_CONVERT_EXPR.

2024-03-16  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/114329
	* gimple-lower-bitint.cc (struct bitint_large_huge): Declare
	build_bit_field_ref method.
	(bitint_large_huge::build_bit_field_ref): New method.
	(bitint_large_huge::lower_mergeable_stmt): Use it.

	* gcc.dg/bitint-101.c: New test.