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

[Jessica Paquette via llvm-commits]

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+# Test the assertion failure in llvm-cov with empty bitmap (#78922)
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ornata wrote:

Can you check for the specific assert?

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