ConstantFoldInstruction: avoid wasted calls to ConstantFoldConstantExpression

[escha]

Summary

The loop here calls ConstantFoldConstantExpression before knowing if all the operands are constant, which can waste a lot of time. Cuts time spent in ConstantFoldInstruction (and callees) by ~20% in our tests. Should be NFC.

[escha]

Use llvm::all_of.

[majnemer]

LGTM with nits.

lib/Analysis/ConstantFolding.cpp

986

I don't think you need the llvm:: qualifier.
Also, I think clang-format would not have a space between the capture and the parameter.

lib/Analysis/ConstantFolding.cpp

 
   // Scan the operand list, checking to see if they are all constants, if so,
   // hand off to ConstantFoldInstOperandsImpl.
+  if (!llvm::all_of(I->operands(), [] (Use &U) { return isa<Constant>(U); }))
+    return nullptr;
+
   SmallVector<Constant*, 8> Ops;
   for (User::op_iterator i = I->op_begin(), e = I->op_end(); i != e; ++i) {
-    Constant *Op = dyn_cast<Constant>(*i);
-    if (!Op)
-      return nullptr;  // All operands not constant!
-
+    Constant *Op = cast<Constant>(*i);
     // Fold the Instruction's operands.
     if (ConstantExpr *NewCE = dyn_cast<ConstantExpr>(Op))
       Op = ConstantFoldConstantExpression(NewCE, DL, TLI);