As mentioned in the PR and discussed on IRC, the following patch is the
patch that fixes the originally reported issue.
We have there because of the premature bitfield comparison -> BIT_FIELD_REF
optimization:
  s$s4_19 = 0;
  s.s4 = s$s4_19;
  _10 = BIT_FIELD_REF <s, 8, 0>;
  _13 = _10 & 8;
and no other s fields are initialized.  If they would be all initialized with
constants, then my earlier PR93582 bitfield handling patches would handle it
already, but if at least one bit we ignore after the BIT_AND_EXPR masking
is not initialized or is initialized earlier to non-constant, we aren't able
to look through it until combine, which is too late for the warnings on the
dead code.
This patch handles BIT_AND_EXPR where the first operand is a SSA_NAME
initialized with a memory load and second operand is INTEGER_CST, by trying
a partial def lookup after pushing the ranges of 0 bits in the mask as
artificial initializers.  In the above case on little-endian, we push
offset 0 size 3 {} partial def and offset 4 size 4 (the result is unsigned
char) and then perform normal partial def handling.
My initial version of the patch failed miserably during bootstrap, because
data->finish (...) called vn_reference_lookup_or_insert_for_pieces
which I believe tried to remember the masked value rather than real for the
reference, or for failed lookup visit_reference_op_load called
vn_reference_insert.  The following version makes sure we aren't calling
either of those functions in the masked case, as we don't know anything
better about the reference from whatever has been discovered when the load
stmt has been visited, the patch just calls vn_nary_op_insert_stmt on
failure with the lhs (apparently calling it with the INTEGER_CST doesn't
work).

2020-03-03  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/93582
	* tree-ssa-sccvn.h (vn_reference_lookup): Add mask argument.
	* tree-ssa-sccvn.c (struct vn_walk_cb_data): Add mask and masked_result
	members, initialize them in the constructor and if mask is non-NULL,
	artificially push_partial_def {} for the portions of the mask that
	contain zeros.
	(vn_walk_cb_data::finish): If mask is non-NULL, set masked_result to
	val and return (void *)-1.  Formatting fix.
	(vn_reference_lookup_pieces): Adjust vn_walk_cb_data initialization.
	Formatting fix.
	(vn_reference_lookup): Add mask argument.  If non-NULL, don't call
	fully_constant_vn_reference_p nor vn_reference_lookup_1 and return
	data.mask_result.
	(visit_nary_op): Handle BIT_AND_EXPR of a memory load and INTEGER_CST
	mask.
	(visit_stmt): Formatting fix.

	* gcc.dg/tree-ssa/pr93582-10.c: New test.
	* gcc.dg/pr93582.c: New test.
	* gcc.c-torture/execute/pr93582.c: New test.