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

[Matt Arsenault via llvm-commits]

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@@ -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>
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arsenm wrote:

I'd rather avoid introducing either of these 2 options

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