[SPIR-V] Do instruction selection for G_BITCAST on an earlier stage (#114216)

This PR implements instruction selection for G_BITCAST on an earlier
stage to avoid MachineVerifier complains on subtle semantics difference
between G_BITCAST and OpBitcast.

We do instruction selections for OpBitcast after IR Translation instead
of calling MIB.buildBitcast() generating the general op code G_BITCAST,
because when MachineVerifier validates G_BITCAST we see a check of a
kind: 'if Source Type is equal to Destination Type then report error
"bitcast must change the type"'. This doesn't take into account the
notion of a typed pointer that is important for SPIR-V where a user may
and should use bitcast between pointers with different pointee types
(https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#OpBitcast).

It's important for correct lowering in SPIR-V, because interpretation of
the data type is not left to instructions that utilize the pointer, but
encoded by the pointer declaration, and the SPIRV target can and must
handle the declaration and use of pointers that specify the type of data
they point to.

It's not feasible to improve validation of G_BITCAST using just
information provided by low level types of source and destination.
Therefore we don't produce G_BITCAST as the general op code with
semantics different from OpBitcast, but rather lower to OpBitcast
immediately.

See discussion in https://github.com/llvm/llvm-project/pull/110270 for
even more context.

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