PTX: Use external symbols to keep track of params and locals. This also fixes

PTX: Use external symbols to keep track of params and locals. This also fixes
a couple of outstanding issues with frame objects occuring as instruction
operands.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@140616 91177308-0d34-0410-b5e6-96231b3b80d8