[Coverage] getMaxBitmapSize: Scan `max(BitmapIdx)` instead of the last `Decision` (#78963)

In `CoverageMapping.cpp:getMaxBitmapSize()`, 
this assumed that the last `Decision` has the maxmum `BitmapIdx`.

Let it scan `max(BitmapIdx)`.

Note that `<=` is used insted of `<`, because `BitmapIdx == 0` is valid
and `MaxBitmapID` is `unsigned`. `BitmapIdx` is unique in the record.

Fixes #78922