[llvm] [NVPTX][SelectionDAG] Add IMAD combine rules + infra to disable default SelectionDAG rules for testing (PR #121724)

[Matt Arsenault via llvm-commits]

================
@@ -153,6 +153,13 @@ static cl::opt<bool> EnableVectorFCopySignExtendRound(
     "combiner-vector-fcopysign-extend-round", cl::Hidden, cl::init(false),
     cl::desc(
         "Enable merging extends and rounds into FCOPYSIGN on vector types"));
+
+static cl::opt<bool>
----------------
arsenm wrote:

There is code that relies on the folds in getNode occurring, introducing a way to disable it just introduces a way to subtly break things. SelectionDAG testing is hard, and that's part of why we should move to globalisel. 

https://github.com/llvm/llvm-project/pull/121724