Function is_hexagon_register_span 

pub fn is_hexagon_register_span(possible_label: &str) -> bool

Checks if a potential label is actually a Hexagon register span notation.

Hexagon assembly uses register span notation like r1:0, V5:4.w, p1:0 etc. These follow the pattern: [letter][digit(s)]:[digit(s)][optional_suffix]

Returns true if the string matches a valid Hexagon register span pattern.