Handle sibcalls with aggregate returns

Handle sibcalls with aggregate returns

We treated this g as a sibling call to f:

      int f (int);
      int g (void) { return f (1); }

but not this one:

      struct s { int i; };
      struct s f (int);
      struct s g (void) { return f (1); }

We treated them both as sibcalls on x86 before the first patch for PR36326,
so I suppose this is a regression of sorts from 4.3.

The patch allows function returns to be local aggregate variables as well
as gimple registers.

gcc/
	* tree-tailcall.c (process_assignment): Simplify the check for
	a valid copy, allowing the source to be a local variable as
	well as an SSA name.
	(find_tail_calls): Allow copies between local variables to follow
	the call.  Allow the result to be stored in any local variable,
	even if it's an aggregate.
	(eliminate_tail_call): Check whether the result is an SSA name
	before updating its SSA_NAME_DEF_STMT.

gcc/testsuite/
	* gcc.dg/tree-ssa/tailcall-7.c: New test.

From-SVN: r242668