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

[NAKAMURA Takumi via llvm-commits]

chapuni wrote:

```llvm/lib/ProfileData/Coverage/CoverageMapping.cpp:232: Expected<BitVector> llvm::coverage::CounterMappingContext::evaluateBitmap(const CounterMappingRegion *) const: Assertion `ID + SizeInBytes <= BitmapBytes.size() && "BitmapBytes overrun"' failed.``` in https://buildkite.com/llvm-project/github-pull-requests/builds/31563



https://github.com/llvm/llvm-project/pull/78963