[amdgpu] Skip non-instruction values in CF user tracing.

Summary:
- CF users won't be non-instruction values. Skip them to save the
  compilation time. It's especially true when there are multiple
  functions in that module, where, says, a constant may be used in most
  functions. The current CF user tracing adds significant overhead.

Reviewers: alex-t, rampitec

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72174

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -11023,6 +11023,8 @@ SITargetLowering::getRegClassFor(MVT VT, bool isDivergent) const {
 }
 
 static bool hasCFUser(const Value *V, SmallPtrSet<const Value *, 16> &Visited) {
+  if (!isa<Instruction>(V))
+    return false;
   if (!Visited.insert(V).second)
     return false;
   bool Result = false;