[llvm] [GlobalISel] Add computeNumSignBits for ASHR (PR #139503)

[Matt Arsenault via llvm-commits]

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@@ -960,6 +968,64 @@ unsigned GISelValueTracking::computeNumSignBits(Register R, unsigned Depth) {
   return computeNumSignBits(R, DemandedElts, Depth);
 }
 
+std::optional<ConstantRange> GISelValueTracking::getValidShiftAmountRange(
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arsenm wrote:

Does this really need to return the full range? The IR version seems to only compute if it's known in range 

https://github.com/llvm/llvm-project/pull/139503