tree-optimization/101769 - tail recursion creates possibly infinite loop

This makes tail recursion optimization produce a loop structure
manually rather than relying on loop fixup.  That also allows the
loop to be marked as finite (it would eventually blow the stack
if it were not).

2021-08-04  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101769
	* tree-tailcall.c (eliminate_tail_call): Add the created loop
	for the first recursion and return it via the new output parameter.
	(optimize_tail_call): Pass through new output param.
	(tree_optimize_tail_calls_1): After creating all latches,
	add the created loop to the loop tree.  Do not mark loops for fixup.

	* g++.dg/tree-ssa/pr101769.C: New testcase.

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