[NFC][NVPTX] remove truncating c-style cast (#85889)

While a stack size large enough to cause this truncation to be a problem
would certainly cause other issues and not produce a valid program
anyway, this cast is triggering our Coverity static analysis. Removing
it seems cleaner.

Author: AlexMaclean

llvm/lib/Target/NVPTX/NVPTXAsmPrinter.cpp

@@ -1719,7 +1719,7 @@ void NVPTXAsmPrinter::setAndEmitFunctionVirtualRegisters(
 
   // Emit the Fake Stack Object
   const MachineFrameInfo &MFI = MF.getFrameInfo();
-  int NumBytes = (int) MFI.getStackSize();
+  int64_t NumBytes = MFI.getStackSize();
   if (NumBytes) {
     O << "\t.local .align " << MFI.getMaxAlign().value() << " .b8 \t"
       << DEPOTNAME << getFunctionNumber() << "[" << NumBytes << "];\n";