PHINode: introduce setIncomingValueForBlock() function, and use it.

PHINode: introduce setIncomingValueForBlock() function, and use it.

Summary:
There is PHINode::getBasicBlockIndex() and PHINode::setIncomingValue()
but no function to replace incoming value for a specified BasicBlock*
predecessor.
Clearly, there are a lot of places that could use that functionality.

Reviewer: craig.topper, lebedev.ri, Meinersbur, kbarton, fhahn
Reviewed By: Meinersbur, fhahn
Subscribers: fhahn, hiraditya, zzheng, jsji, llvm-commits
Tag: LLVM
Differential Revision: https://reviews.llvm.org/D63338

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@363566 91177308-0d34-0410-b5e6-96231b3b80d8