Fix for PR19099 - NVPTX produces invalid symbol names.

This is a more thorough fix for the issue than r203483. An IR pass will run
before NVPTX codegen to make sure there are no invalid symbol names that can't
be consumed by the ptxas assembler.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205212 91177308-0d34-0410-b5e6-96231b3b80d8