Add a const version of findNearestCommonDominator to PostDom for convenience.

PR16283.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183629 91177308-0d34-0410-b5e6-96231b3b80d8
tblgen: always lookup values from the original vector as it could be grown under our feet.

PR16281.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183630 91177308-0d34-0410-b5e6-96231b3b80d8