hdl.ast: fix range handling in Shape.cast.

[mwkmwkmwk]

Fixes #803.

[whitequark]

LGTM

[codecov]

Codecov Report

Merging #804 (a86e392) into main (a6e33ab) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #804   +/-   ##
=======================================
  Coverage   82.04%   82.04%           
=======================================
  Files          50       50           
  Lines        6869     6869           
  Branches     1672     1672           
=======================================
  Hits         5636     5636           
+ Misses       1037     1036    -1     
- Partials      196      197    +1     

Impacted Files	Coverage Δ
amaranth/hdl/ast.py	89.50% <100.00%> (ø)

... and 1 file with indirect coverage changes

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