[LOCAL, gcc]: Cherry-pick mainline patch to resolve MB kernel panic

Cherry-pick backend optimization patch from gcc HEAD which 'resolves' a kernel
panic for microblaze when compiled with -Os

Upstream HEAD (soon to be gcc 4.9) does not exhibt this error any longer,
and this patch when applied as a workaround on the 4.8 branch also hides the
kernel panic resulting from incorrect branch-delay slot filling.

        * tree-ssa-threadedge.c (thread_around_empty_block): Remove
        checks for the number of predecessors and successors allowed.
        * tree-ssa-threadupdate.c (mark_threaded_blocks): Ignore requests
        which require copying a joiner block if there is a request which
        is a subpath that requires no joiner block copying.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@202054 138bc75d-0d04-0410-961f-82ee72b054a4

Signed-off-by: David Holsgrove <david.holsgrove@xilinx.com>