GCC caches the whether a function is a leaf in crtl->is_leaf. Using this
in the backend is best as leaf_function_p may not work correctly (eg. while
emitting prolog or epilog code).  There are many reads of crtl->is_leaf
before it is initialized.  Many targets do in targetm.frame_pointer_required
(eg. arm, aarch64, i386, mips, sparc), which is called before register 
allocation by ira_setup_eliminable_regset and sched_init.

Additionally, SHRINK_WRAPPING_ENABLED calls targetm.have_simple_return,
which evaluates the condition of the simple_return instruction.  On ARM
this results in a call to use_simple_return_p which requires crtl->is_leaf
to be set correctly.

To fix this, initialize crtl->is_leaf in ira_setup_eliminable_regset and
early on in ira.  A bootstrap did not find any uninitialized reads of
crtl->is_leaf on Thumb-2.  A follow-up patch will remove incorrect uses
of leaf_function_p from the ARM backend.

    gcc/
	* gcc/ira.c (ira_setup_eliminable_regset): Initialize crtl->is_leaf.
	(ira): Move initialization of crtl->is_leaf earlier.

From-SVN: r243347