Add assert to avoid unexpected situation

[XinWang10]

Summary

In line 68, func getBlockFreqRelativeToEntryBlock assume getEntryFreq() is
non-zero, but the function could return 0, add assert to confirm it.

[HaohaiWen]

LGTM

llvm/include/llvm/CodeGen/MachineBlockFrequencyInfo.h

public:
   /// For example, to get the frequency of a block relative to the entry block,
   /// divide the integral value returned by this function (the
   /// BlockFrequency::getFrequency() value) by getEntryFreq().
   BlockFrequency getBlockFreq(const MachineBasicBlock *MBB) const;
 
   /// Compute the frequency of the block, relative to the entry block.
   /// This API assumes getEntryFreq() is non-zero.
   float getBlockFreqRelativeToEntryBlock(const MachineBasicBlock *MBB) const {
+    assert(getEntryFreq() != 0 && "getEntryFreq() should not return 0 here!");
     return getBlockFreq(MBB).getFrequency() * (1.0f / getEntryFreq());
   }
 
   std::optional<uint64_t>
   getBlockProfileCount(const MachineBasicBlock *MBB) const;
   std::optional<uint64_t> getProfileCountFromFreq(uint64_t Freq) const;
 
   bool isIrrLoopHeader(const MachineBasicBlock *MBB) const;