[AsmPrint] Correctly factor function entry count when dumping MBB frequencies (#67826)

The goal in #66818 was to capture function entry counts, but those are not the same as the frequency of the entry (machine) basic block. This fixes that, and adds explicit profiles to the test.

We also increase the precision of `MachineBlockFrequencyInfo::getBlockFreqRelativeToEntryBlock` to double. Existing code uses it as float so should be unaffected.