[SimplifyCFG] mergeConditionalStoreToAddress(): consider cost, not instruction count

Summary:
As it can be see in the changed test, while `div` is really costly,
we were speculating it. This does not seem correct.

Also, the old code would run for every single insturuction in BB,
instead of eagerly bailing out as soon as there are too many instructions.

This function still has a problem that `PHINodeFoldingThreshold` is
per-basic-block, while it should be for all the basic blocks.

Reviewers: efriedma, craig.topper, dmgreen, jmolloy

Reviewed By: jmolloy

Subscribers: xbolva00, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67315

llvm-svn: 372255