[InstCombine] add control for SimplifyDemandedVectorElts depth limit (#113717)

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

We find it helpful to be able to customize this for compile time
reasons.

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) {