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

[NAKAMURA Takumi via llvm-commits]

chapuni wrote:

https://lab.llvm.org/buildbot/#/builders/16/builds/60018
> error: failed to load coverage: 'llvm/test/tools/llvm-cov/Inputs/mcdc-maxbs.o': profile uses zlib compression but the profile reader was built without zlib support

I'll rework.

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