gimple-fold: Handle bitfields in fold_const_aggregate_ref_1 [PR93121]

gimple-fold: Handle bitfields in fold_const_aggregate_ref_1 [PR93121]

When working on __builtin_bit_cast that needs to handle bitfields too,
I've made the following change to handle at least some bitfields in
fold_const_aggregate_ref_1 (those that have integral representative).
It already handles some, but only those that start and end at byte
boundaries.

2020-07-20  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/93121
	* gimple-fold.c (fold_const_aggregate_ref_1): For COMPONENT_REF
	of a bitfield not aligned on byte boundaries try to
	fold_ctor_reference DECL_BIT_FIELD_REPRESENTATIVE if any and
	adjust it depending on endianity.

	* gcc.dg/tree-ssa/pr93121-2.c: New test.