[llvm] Use BlockFrequency type in more places (NFC) (PR #68266)

[Matthias Braun via llvm-commits]

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@@ -26,10 +26,11 @@ class BlockFrequency {
   uint64_t Frequency;
 
 public:
-  BlockFrequency(uint64_t Freq = 0) : Frequency(Freq) { }
+  BlockFrequency() : Frequency(0) {}
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MatzeB wrote:

I changed all the code in this patch to use an explicit `BlockFrequency(0)`. However in the end I could not remove the default constructor as it breaks the use as a `DenseMap` value  (things like `DenseMap::FindAndConstruct(key)` require a default constructor for the value type).

https://github.com/llvm/llvm-project/pull/68266