[llvm] [NVPTX] Improve support for rsqrt.approx (PR #89417)

[Alex MacLean via llvm-commits]

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@@ -30,6 +30,10 @@ using namespace llvm;
 #define DEBUG_TYPE "nvptx-isel"
 #define PASS_NAME "NVPTX DAG->DAG Pattern Instruction Selection"
 
+static cl::opt<bool>
+    DisableRsqrtOpt("nvptx-disable-rsqrt-opt", cl::init(false), cl::Hidden,
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AlexMaclean wrote:

hmm, what about `-nvptx-rsqrt-approx-opt` or `-nvptx-rsqrt-approx-folding`? I think it would be good to make clear that this controls optimizing `sqrt` to `rsqrt` not whether `rsqrt` will be emitted at all. 

https://github.com/llvm/llvm-project/pull/89417