openmp: Fix up *_reduction clause handling with UDRs on PARM_DECLs [PR101167]

openmp: Fix up *_reduction clause handling with UDRs on PARM_DECLs [PR101167]

The following testcase FAILs, because the UDR combiner is invoked incorrectly.
lower_omp_rec_clauses expects that when it sets
DECL_VALUE_EXPR/DECL_HAS_VALUE_EXPR_P
for both the placeholder and the var that everything will be properly
regimplified, but as the variable in question is a PARM_DECL rather than
VAR_DECL, lower_omp_regimplify_p doesn't say that it should be regimplified
and so it is not.

2021-06-23  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/101167
	* omp-low.c (lower_omp_regimplify_p): Regimplify also PARM_DECLs
	and RESULT_DECLs that have DECL_HAS_VALUE_EXPR_P set.

	* testsuite/libgomp.c-c++-common/task-reduction-15.c: New test.

(cherry picked from commit 679506c3830ea1a93c755413609bfac3538e2cbd)