This operation extracts a number of bits at a given offset and sign or
zero extends them, which is done by emitting it as a left shift followed
by a right shift.
This is being added for use in clang for C++ structured bindings of
bitfields that have offset or size that aren't a byte multiple. A new
operation is being added, instead of shifts being used directly, as it
makes correctly handling it in optimisations (which will be done in a
later patch) much easier.
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