R600: Short circuit alloca check if address space isn't private.

Skip calling GetUnderlyingObject in cases where it obviously
isn't from an alloca. This should only be a compile time improvement.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213229 91177308-0d34-0410-b5e6-96231b3b80d8