tree-optimization/111779 - Handle some BIT_FIELD_REFs in SRA

tree-optimization/111779 - Handle some BIT_FIELD_REFs in SRA

The following handles byte-aligned, power-of-two and byte-multiple
sized BIT_FIELD_REF reads in SRA.  In particular this should cover
BIT_FIELD_REFs created by optimize_bit_field_compare.

For gcc.dg/tree-ssa/ssa-dse-26.c we now SRA the BIT_FIELD_REF
appearing there leading to more DSE, fully eliding the aggregates.

This results in the same false positive -Wuninitialized as the
older attempt to remove the folding from optimize_bit_field_compare,
fixed by initializing part of the aggregate unconditionally.

	PR tree-optimization/111779
gcc/
	* tree-sra.cc (sra_handled_bf_read_p): New function.
	(build_access_from_expr_1): Handle some BIT_FIELD_REFs.
	(sra_modify_expr): Likewise.
	(make_fancy_name_1): Skip over BIT_FIELD_REF.

gcc/fortran/
	* trans-expr.cc (gfc_trans_assignment_1): Initialize
	lhs_caf_attr and rhs_caf_attr codimension flag to avoid
	false positive -Wuninitialized.

gcc/testsuite/
	* gcc.dg/tree-ssa/ssa-dse-26.c: Adjust for more DSE.
	* gcc.dg/vect/vect-pr111779.c: New testcase.