Fix shrink-wrapping bug (PR67778, PR68634)

Fix shrink-wrapping bug (PR67778, PR68634)

After shrink-wrapping has found the "tightest fit" for where to place
the prologue, it tries move it earlier (so that frame saves are run
earlier) -- but without copying any more basic blocks.

Unfortunately a candidate block we select can be inside a loop, and we
will still allow it (because the loop always exits via our previously
chosen block).  We can do that just fine if we make a duplicate of the
block, but we do not want to here.

So we need to detect this situation.  We can place the prologue at a
previous block PRE only if PRE dominates every block reachable from
it, because then we will never need to duplicate that block (it will
always be executed with prologue).

2015-12-11  Segher Boessenkool  <segher@kernel.crashing.org>

	PR rtl-optimization/67778
	PR rtl-optimization/68634
	* shrink-wrap.c (try_shrink_wrapping): Add a comment about why we want
	to put the prologue earlier.  When determining if an earlier block is
	suitable, make sure it dominates every block reachable from it.

From-SVN: r231552