Support AddrSpaceCast in ConstantExpr::getAsInstruction.

It's handled similarly to the other casts. CastInst::Create already knows how
to handle it.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199565 91177308-0d34-0410-b5e6-96231b3b80d8