ipa: special pass-through op for Fortran strides

when Fortran functions pass array descriptors they receive as a
parameter to another function, they actually rebuild it.  Thanks to
work done mainly by Feng, IPA-CP can already handle the cases when
they pass directly the values loaded from the original descriptor.
Unfortunately, perhaps the most important one, stride, is first
checked against zero and is replaced with one in that case:

  _12 = *a_11(D).dim[0].stride;
  if (_12 != 0)
    goto <bb 4>; [50.00%]
  else
    goto <bb 3>; [50.00%]

  <bb 3>
    // empty BB
  <bb 4>
  # iftmp.22_9 = PHI <_12(2), 1(3)>
   ...
   parm.6.dim[0].stride = iftmp.22_9;
   ...
   __x_MOD_foo (&parm.6, b_31(D));

in the most important and hopefully common cases, the incoming value
is already 1 and we fail to propagate it.

I would therefore like to propose the following way of encoding this
situation in pass-through jump functions using using ASSERTT_EXPR
operation code meaning that if the incoming value is the same as the
"operand" in the jump function, it is passed on, otherwise the result
is unknown.  This of course captures only the single (but most
important) case but is an improvement and does not need enlarging the
jump function structure and is simple to pattern match.  Encoding that
zero needs to be changed to one would need another field and matching
it would be slightly more complicated too.

gcc/
2020-06-12  Martin Jambor  <mjambor@suse.cz>

	* ipa-prop.h (ipa_pass_through_data): Expand comment describing
	operation.
	* ipa-prop.c (analyze_agg_content_value): Detect new special case and
	encode it as ASSERT_EXPR.
	* ipa-cp.c (values_equal_for_ipcp_p): Move before
	ipa_get_jf_arith_result.
	(ipa_get_jf_arith_result): Special case ASSERT_EXPR.

gcc/testsuite/
2020-06-12  Martin Jambor  <mjambor@suse.cz>
	* gfortran.dg/ipcp-array-2.f90: New test.

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