debug/104337 - avoid messing with the abstract origin chain in NRV

debug/104337 - avoid messing with the abstract origin chain in NRV

The following avoids NRV from massaging DECL_ABSTRACT_ORIGIN after
variable creation since NRV runs _after_ the function was inlined and thus
affects the inlined variables copy indirectly.  We may adjust the abstract
origin of a variable only at the point we create it, not further along the
path since otherwise the (new) invariant that the abstract origin is always
the ultimate origin cannot be maintained.

The intent of what NRV does is OK I guess and it may improve the debug
experience.  But I also notice we do

  SET_DECL_VALUE_EXPR (found, result);
  DECL_HAS_VALUE_EXPR_P (found) = 1;

the code is there since the merge from tree-ssa which added tree-nrv.c.

Jakub added the DECL_VALUE_EXPR in g:938650d8fddb878f623e315f0b7fd94b217efa96
and Jason added the abstract origin setting conditional in g:7716876bbd3a

The follwoing takes the radical approach and remove the attempt
to "optimize" the debug info.

The gdb testsuites show no regressions.

2022-02-03  Richard Biener  <rguenther@suse.de>

	PR debug/104337
	* tree-nrv.cc (pass_nrv::execute): Remove tieing result and found
	together via DECL_ABSTRACT_ORIGIN.

	* gcc.dg/debug/pr104337.c: New testcase.