[InstCombine] Add convenience helper tryGetLog2; NFC

This just encapsulates the common pattern:

```
if (takeLog2(..., /*DoFold=*/false)) {
    Value * Log2 = takeLog2(..., /*DoFold=*/true);
    ...
}
```

Author: goldsteinn

llvm/include/llvm/Transforms/InstCombine/InstCombiner.h

@@ -201,6 +201,12 @@ class LLVM_LIBRARY_VISIBILITY InstCombiner {
   // takeLog2 will succeed, otherwise it may create stray instructions.
   Value *takeLog2(Value *Op, unsigned Depth, bool AssumeNonZero, bool DoFold);
 
+  Value *tryGetLog2(Value *Op, bool AssumeNonZero) {
+    if (takeLog2(Op, /*Depth=*/0, AssumeNonZero, /*DoFold=*/false))
+      return takeLog2(Op, /*Depth=*/0, AssumeNonZero, /*DoFold=*/true);
+    return nullptr;
+  }
+
   /// Return nonnull value if V is free to invert under the condition of
   /// WillInvertAllUses.
   /// If Builder is nonnull, it will return a simplified ~V.

llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp

@@ -528,19 +528,13 @@ Instruction *InstCombinerImpl::visitMul(BinaryOperator &I) {
   //        (shl Op1, Log2(Op0))
   //    if Log2(Op1) folds away ->
   //        (shl Op0, Log2(Op1))
-  if (takeLog2(Op0, /*Depth*/ 0, /*AssumeNonZero*/ false,
-               /*DoFold*/ false)) {
-    Value *Res = takeLog2(Op0, /*Depth*/ 0, /*AssumeNonZero*/ false,
-                          /*DoFold*/ true);
+  if (Value *Res = tryGetLog2(Op0, /*AssumeNonZero=*/false)) {
     BinaryOperator *Shl = BinaryOperator::CreateShl(Op1, Res);
     // We can only propegate nuw flag.
     Shl->setHasNoUnsignedWrap(HasNUW);
     return Shl;
   }
-  if (takeLog2(Op1, /*Depth*/ 0, /*AssumeNonZero*/ false,
-               /*DoFold*/ false)) {
-    Value *Res = takeLog2(Op1, /*Depth*/ 0, /*AssumeNonZero*/ false,
-                          /*DoFold*/ true);
+  if (Value *Res = tryGetLog2(Op0, /*AssumeNonZero=*/false)) {
     BinaryOperator *Shl = BinaryOperator::CreateShl(Op0, Res);
     // We can only propegate nuw flag.
     Shl->setHasNoUnsignedWrap(HasNUW);
@@ -1506,13 +1500,9 @@ Instruction *InstCombinerImpl::visitUDiv(BinaryOperator &I) {
   }
 
   // Op1 udiv Op2 -> Op1 lshr log2(Op2), if log2() folds away.
-  if (takeLog2(Op1, /*Depth*/ 0, /*AssumeNonZero*/ true,
-               /*DoFold*/ false)) {
-    Value *Res = takeLog2(Op1, /*Depth*/ 0,
-                          /*AssumeNonZero*/ true, /*DoFold*/ true);
+  if (Value *Res = tryGetLog2(Op1, /*AssumeNonZero=*/true))
     return replaceInstUsesWith(
         I, Builder.CreateLShr(Op0, Res, I.getName(), I.isExact()));
-  }
 
   return nullptr;
 }