[ldist] Don't add lib calls with -fno-tree-loop-distribute-patterns

As mentioned in PR56888 comment 21:
...
-fno-tree-loop-distribute-patterns is the reliable way to not
transform loops into library calls.
...

However, since commit 6f966f06146 ("ldist: Recognize strlen and rawmemchr like
loops") a strlen or rawmemchr library call may be introduced by ldist.

This caused regressions in testcases
gcc.c-torture/execute/builtins/strlen{,-2,-3}.c for nvptx.

Fix this by not calling transform_reduction_loop from
loop_distribution::execute for -fno-tree-loop-distribute-patterns.

Tested regressing test-cases as well as gcc.dg/tree-ssa/ldist-*.c on
nvptx.

gcc/ChangeLog:

2022-01-31  Tom de Vries  <tdevries@suse.de>

	* tree-loop-distribution.cc (generate_reduction_builtin_1): Check for
	-ftree-loop-distribute-patterns.
	(loop_distribution::execute): Don't call transform_reduction_loop for
	-fno-tree-loop-distribute-patterns.

gcc/testsuite/ChangeLog:

2022-01-31  Tom de Vries  <tdevries@suse.de>

	* gcc.dg/tree-ssa/ldist-strlen-4.c: New test.

---
 gcc/testsuite/gcc.dg/tree-ssa/ldist-strlen-4.c | 17 +++++++++++++++++
 gcc/tree-loop-distribution.cc                  |  5 ++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ldist-strlen-4.c b/gcc/testsuite/gcc.dg/tree-ssa/ldist-strlen-4.c
new file mode 100644
index 000000000000..eafb37e84fc3
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ldist-strlen-4.c
@@ -0,0 +1,17 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -ftree-loop-distribution -fno-tree-loop-distribute-patterns -fdump-tree-ldist-details" } */
+/* { dg-final { scan-tree-dump-not "generated strlen" "ldist" } } */
+
+/* Copied from gcc/testsuite/gcc.c-torture/execute/builtins/lib/strlen.c.  */
+
+__SIZE_TYPE__
+foo (const char *s)
+{
+  __SIZE_TYPE__ i;
+
+  i = 0;
+  while (s[i] != 0)
+    i++;
+
+  return i;
+}
diff --git a/gcc/tree-loop-distribution.cc b/gcc/tree-loop-distribution.cc
index 6fe59cd56855..c7b428572636 100644
--- a/gcc/tree-loop-distribution.cc
+++ b/gcc/tree-loop-distribution.cc
@@ -3290,6 +3290,8 @@ generate_reduction_builtin_1 (loop_p loop, gimple_seq &seq,
 			      tree reduction_var_old, tree reduction_var_new,
 			      const char *info, machine_mode load_mode)
 {
+  gcc_assert (flag_tree_loop_distribute_patterns);
+
   /* Place new statements before LOOP.  */
   gimple_stmt_iterator gsi = gsi_last_bb (loop_preheader_edge (loop)->src);
   gsi_insert_seq_after (&gsi, seq, GSI_CONTINUE_LINKING);
@@ -3773,7 +3775,8 @@ loop_distribution::execute (function *fun)
       if (niters == NULL_TREE || niters == chrec_dont_know)
 	{
 	  datarefs_vec.create (20);
-	  if (transform_reduction_loop (loop))
+	  if (flag_tree_loop_distribute_patterns
+	      && transform_reduction_loop (loop))
 	    {
 	      changed = true;
 	      loops_to_be_destroyed.safe_push (loop);