openmp: Fix up handling of reduction clauses on the loop construct [PR102431]

openmp: Fix up handling of reduction clauses on the loop construct [PR102431]

We were using unshare_expr and walk_tree_without_duplicate replacement
of the placeholder vars.  The OMP_CLAUSE_REDUCTION_{INIT,MERGE} can contain
other trees that need to be duplicated though, e.g. BLOCKs referenced in
BIND_EXPR(s), or local VAR_DECLs.  This patch uses the inliner code to copy
all of that.  There is a slight complication that those local VAR_DECLs or
placeholders don't have DECL_CONTEXT set, they will get that only when
they are gimplified later on, so this patch sets DECL_CONTEXT for those
temporarily and resets it afterwards.

2021-11-23  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/102431
	* gimplify.c (replace_reduction_placeholders): Remove.
	(note_no_context_vars): New function.
	(gimplify_omp_loop): For OMP_PARALLEL's BIND_EXPR create a new
	BLOCK.  Use copy_tree_body_r with walk_tree instead of unshare_expr
	and replace_reduction_placeholders for duplication of
	OMP_CLAUSE_REDUCTION_{INIT,MERGE} expressions.  Ensure all mentioned
	automatic vars have DECL_CONTEXT set to non-NULL before doing so
	and reset it afterwards for those vars and their corresponding
	vars.

	* c-c++-common/gomp/pr102431.c: New test.
	* g++.dg/gomp/pr102431.C: New test.

(cherry picked from commit 5e9b973bd60185f221222022f56db7df3d92250e)