XQuery: Merge exact and range comparisons

[ChristianGruen]

Examples:

<_>0</_>[.  = 0 and . >= 0]  →  <_>0</_>[.  = 0]
<_>0</_>[. <= 0 or  .  = 0]  →  <_>0</_>[. <= 0]

(1 to 6)[.  = 3 and . <= 3]  →  (1 to 6)[.  = 3]
(1 to 6)[. <= 3 or  .  = 3]  →  (1 to 6)[. <= 3]

<_>X</_>[.  = 'X' and . >= 'W']  →  <_>X</_>[. = 'X']
<_>X</_>[. <= 'W' and .  = 'X']  →  ()