[NVPTX] Assign valid global names

PTX requires that identifiers consist only of [a-zA-Z0-9_$]. The
existing pass already ensured this for globals and this patch adds
the cleanup for functions with local linkage.

However, there was a different problem in the case of collisions
of the adjusted name: The ValueSymbolTable then automatically
appended ".N" with increasing Ns to get a unique name while helping
the ABI demangling. Special case this behavior to omit the dots and
append N directly. This will always give us legal names according
to the PTX requirements.

Differential Revision: https://reviews.llvm.org/D40573

llvm-svn: 319657