[llvm] [NVPTX] improve identifier renaming for PTX (PR #79459)

[Artem Belevich via llvm-commits]

================
@@ -73,10 +74,10 @@ std::string NVPTXAssignValidGlobalNames::cleanUpName(StringRef Name) {
   std::string ValidName;
   raw_string_ostream ValidNameStream(ValidName);
   for (char C : Name) {
-    if (C == '.' || C == '@' || C == '<' || C == '>') {
-      ValidNameStream << "_$_";
-    } else {
+    if (isAlnum(C) || C == '_' || C == '$') {
       ValidNameStream << C;
+    } else {
+      ValidNameStream << "_$_";
----------------
Artem-B wrote:

Ugh. I think we'll have a problem when we have symbols that have different invalid characters in the same place. E.g. `str-` and `str.`. 
I think ideally we need to encode specific symbols instead. So, instead of just `_$_` for all invalid characters, emit ` "$"+hex(char)` which will be unambiguous.

https://github.com/llvm/llvm-project/pull/79459