This is a trivial tweak to the addrec insertion code: insert the increment

This is a trivial tweak to the addrec insertion code: insert the increment
at the bottom of the loop instead of the top.  This reduces the number of
overlapping live ranges a lot, for example, eliminating a spill in an important
loop in 183.equake with linear scan.

I still need to make the exit comparison of the loop use the post-incremented
version of this variable, but this is an easy first step.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12952 91177308-0d34-0410-b5e6-96231b3b80d8