Added panic-on-overflow for rhs of left and right shift expressions.

This includes a slight refactoring of the `cast_shift_rhs` and related
functions in `trans::base`, so that I can call them from much later in
the compiler's control flow (so that we can clearly dilineate where
automatic conversions of the RHS occur, versus where we check it).

The rhs-checking and fallback-masking is generalized to 8- and 16-bit
values, and the fallback-masking is turned on unconditionally.

Fix #10183.

Is this a [breaking-change]?  I would argue it is not; it only adds a
strict definition to what was previously undefined behavior; however,
there might be code that was e.g. assuming that `1_i8 << 17` yields 0.
(This happens in certain contexts and at certain optimization levels.)

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