[CI] add assert-enabled MLIR option (#78)

This deprecates the use of release-build LLVM hosted by the LLVM project, which makes debugging harder for developers.

This PR implements the following solution:
1. Create LLVM release tarballs with assert enabled on our own (using Docker)
2. Host them in our own GitHub repositories
3. Use our LLVM for CI and/or development if `TRITON_USE_ASSERT_ENABLED_LLVM=1` is set.

lib/Target/LLVMIR/LLVMIRTranslation.cpp

@@ -65,7 +65,7 @@ void extractNVVMMetadata(mlir::ModuleOp module,
     // maxntid
     if (op->hasAttr(NVVMMetadataField::MaxNTid)) {
       auto attr = op->getAttr(NVVMMetadataField::MaxNTid);
-      meta.maxntidx = attr.dyn_cast<IntegerAttr>().getInt();
+      meta.maxntidx = attr.dyn_cast<IntegerAttr>().getSInt();
       hasMetadata = true;
     }