Improve code for shifts and bitwise operations.

- JavaScript shifts can be used for operands in certain ranges.

- Coercion from signed-32 to unsigned-32 can be omitted if
  - it is a no-op
  - the context performs the coercion
  - the context is insensitive to the most significant bit

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Review URL: https://codereview.chromium.org/1920293005 .

Author: rakudrama