Refactor setAlreadyUnrolled() and setAlreadyVectorized().

Loop::setAlreadyUnrolled() and
LoopVectorizeHints::setLoopAlreadyUnrolled() both add loop metadata that
stops the same loop from being transformed multiple times. This patch
merges both implementations.

In doing so we fix 3 potential issues:

 * setLoopAlreadyUnrolled() kept the llvm.loop.vectorize/interleave.*
   metadata even though it will not be used anymore. This already caused
   problems such as Change the behavior to the
   one of setAlreadyUnrolled which deletes this loop metadata.

 * setAlreadyUnrolled() used to create a new LoopID by calling
   MDNode::get with nullptr as the first operand, then replacing it by
   the returned references using replaceOperandWith. It is possible
   that MDNode::get would instead return an existing node (due to
   de-duplication) that then gets modified. To avoid, use a fresh
   TempMDNode that does not get uniqued with anything else before
   replacing it with replaceOperandWith.

 * LoopVectorizeHints::matchesHintMetadataName() only compares the
   suffix of the attribute to set the new value for. That is, when
   called with "enable", would erase attributes such as
   "llvm.loop.unroll.enable", "llvm.loop.vectorize.enable" and
   "llvm.loop.distribute.enable" instead of the one to replace.
   Fortunately, function was only called with "isvectorized".

Differential Revision:

git-svn-id: 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed