Reject tail calls that read from an escaped RESULT_DECL (PR90313)

Reject tail calls that read from an escaped RESULT_DECL (PR90313)

In this PR we have two return paths from a function "map".  The common
code sets <result> to the value returned by one path, while the other
path does:

   <retval> = map (&<retval>, ...);

We treated this call as tail recursion, losing the copy semantics
on the value returned by the recursive call.

We'd correctly reject the same thing for variables:

   local = map (&local, ...);

The problem is that RESULT_DECLs didn't get the same treatment.

2019-08-09  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR middle-end/90313
	* tree-tailcall.c (find_tail_calls): Reject calls that might
	read from an escaped RESULT_DECL.

gcc/testsuite/
	PR middle-end/90313
	* g++.dg/torture/pr90313.cc: New test.

From-SVN: r274234