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

[NAKAMURA Takumi via llvm-commits]

chapuni wrote:

The record of this testcase is:
```
sub:
  Decision,File 0, 5:11 -> 5:59 = M:1, C:4
  Branch,File 0, 5:12 -> 5:20 = #5, ((#0 - #1) - #5) [1,3,2]
  Branch,File 0, 5:24 -> 5:32 = #6, (#5 - #6) [3,0,2]
  Branch,File 0, 5:38 -> 5:46 = #7, (#4 - #7) [2,4,0]
  Branch,File 0, 5:50 -> 5:58 = #8, (#7 - #8) [4,0,0]
  Decision,File 1, 1:23 -> 1:47 = M:0, C:2
  Branch,File 1, 1:23 -> 1:33 = #2, (#0 - #2) [1,2,0]
  Branch,File 1, 1:37 -> 1:47 = #3, (#2 - #3) [2,0,0]
```

The current implementation picks up `M:0 C:2`.

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