Fix B031 false positive for usage in mutually exclusive if/else branches

[Fridayai700]

Fixes #465.

Using the groupby() generator in separate branches of an if/else is safe since only one branch executes. Previously, B031 counted all Name references in the loop body regardless of control flow, so this code was incorrectly flagged:

for _, group in itertools.groupby([]):
    if condition:
        list(group)
    else:
        list(group)  # false positive: B031

This refactors the counting logic to handle if/elif/else branches as mutually exclusive, taking the maximum usage count across branches rather than the sum. The change correctly handles:

• Simple if/else (no flag)
• if/elif/else chains (no flag)
• Usage before if + inside if body (flags correctly)
• Usage in both branches + outside (flags correctly)
• Nested loops (always flags, as before)