[SimplifyCFG] When merging conditional stores, don't count the store we're merging...

[SimplifyCFG] When merging conditional stores, don't count the store we're merging against the PHINodeFoldingThreshold

Merging conditional stores tries to check to see if the code is if convertible after the store is moved. But the store hasn't been moved yet so its being counted against the threshold.

The patch adds 1 to the threshold comparison to make sure we don't count the store. I've adjusted a test to use a lower threshold to ensure we still do that conversion with the lower threshold.

Differential Revision: https://reviews.llvm.org/D39570

llvm-svn: 317368