The following patch is first step towards fixing PR93582.
vn_reference_lookup_3 right now punts on anything that isn't byte aligned,
so to be able to lookup a constant bitfield store, one needs to use
the exact same COMPONENT_REF, otherwise it isn't found.

This patch lifts up that that restriction if the bits to be loaded are
covered by a single store of a constant (keeps the restriction so far
for the multiple store case, can tweak that incrementally, but I think
for bisection etc. it is worth to do it one step at a time).

2020-02-13  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/93582
	* fold-const.h (shift_bytes_in_array_left,
	shift_bytes_in_array_right): Declare.
	* fold-const.c (shift_bytes_in_array_left,
	shift_bytes_in_array_right): New function, moved from
	gimple-ssa-store-merging.c, no longer static.
	* gimple-ssa-store-merging.c (shift_bytes_in_array): Move
	to gimple-ssa-store-merging.c and rename to shift_bytes_in_array_left.
	(shift_bytes_in_array_right): Move to gimple-ssa-store-merging.c.
	(encode_tree_to_bitpos): Use shift_bytes_in_array_left instead of
	shift_bytes_in_array.
	(verify_shift_bytes_in_array): Rename to ...
	(verify_shift_bytes_in_array_left): ... this.  Use
	shift_bytes_in_array_left instead of shift_bytes_in_array.
	(store_merging_c_tests): Call verify_shift_bytes_in_array_left
	instead of verify_shift_bytes_in_array.
	* tree-ssa-sccvn.c (vn_reference_lookup_3): For native_encode_expr
	/ native_interpret_expr where the store covers all needed bits,
	punt on PDP-endian, otherwise allow all involved offsets and sizes
	not to be byte-aligned.

	* gcc.dg/tree-ssa/pr93582-1.c: New test.
	* gcc.dg/tree-ssa/pr93582-2.c: New test.
	* gcc.dg/tree-ssa/pr93582-3.c: New test.