Fix grammar in addrspacecast description

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194785 91177308-0d34-0410-b5e6-96231b3b80d8
Add instcombine visitor for addrspacecast

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194786 91177308-0d34-0410-b5e6-96231b3b80d8