[InstCombine] add control for SimplifyDemandedVectorElts depth limit

Allows customizing the depth of the recursive search on vectors that
InstCombine does when looking for unused elements.

Author: Prince781

llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

@@ -30,6 +30,12 @@ static cl::opt<bool>
                              "SimplifyDemandedBits() are consistent"),
                     cl::Hidden, cl::init(false));
 
+static cl::opt<unsigned> SimplifyDemandedVectorEltsDepthLimit(
+    "instcombine-simplify-vector-elts-depth",
+    cl::desc(
+        "Depth limit when simplifying vector instructions and their operands"),
+    cl::Hidden, cl::init(10));
+
 /// Check to see if the specified operand of the specified instruction is a
 /// constant integer. If so, check to see if there are any bits set in the
 /// constant that are not demanded. If so, shrink the constant and return true.
@@ -1432,7 +1438,7 @@ Value *InstCombinerImpl::SimplifyDemandedVectorElts(Value *V,
   }
 
   // Limit search depth.
-  if (Depth == 10)
+  if (Depth == SimplifyDemandedVectorEltsDepthLimit)
     return nullptr;
 
   if (!AllowMultipleUsers) {