Respect address space sizes in isEliminableCastPair.

This avoids constant folding bitcast/ptrtoint/inttoptr combinations
that have illegal bitcasts between differently sized address spaces.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187455 91177308-0d34-0410-b5e6-96231b3b80d8