[tautschnig]

For a non-constant index, extractbit(src, index) is encoded as bit 0
of src shifted right by index. The index was converted to the type of
the source with a typecast_exprt, which converts the value: for a
fixedbv source it scales the index by 2^fraction_bits, for a c_bool
source it maps any non-zero index to 1, and for a source narrower
than the index it truncates the index, so that an out-of-range index
such as 256 for an 8-bit source selects bit 0.

Instead, zero-extend both the source and the index to the wider of the
two widths. Then an out-of-range index shifts out all bits and the
result is false. (The SAT back end leaves the result unconstrained for
out-of-range indices.)

C programs do not produce such expressions, but other front ends do.

Fixes #9367

Found by Jeroen Robben's specification-conformance pipeline (https://reports.robben.io/methodology/).

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