Split some code out to a helper function (FindReusablePredBB) and add a doxygen comment.
Split some code out to a helper function (FindReusablePredBB)
and add a doxygen comment.

Cache the phi entry to avoid doing tons of 
PHINode::getBasicBlockIndex calls in the common case.

On my insane testcase from re2c, this speeds up CGP from
617.4s to 7.9s (78x).



git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@96083 91177308-0d34-0410-b5e6-96231b3b80d8

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