[Libomptarget] Rework image checking further (#76120)

Summary:
In the future, we may have more checks for different kinds of inputs,
e.g. SPIR-V. This patch simply reworks the handling to be more generic
and do the magic detection up-front. The checks inside the routines are
now asserts so we don't spend time checking this stuff over and over
again.

This patch also tweaked the bitcode check. I used a different function
to get the Lazy-IR module now, as it returns the raw expected value
rather than the SM diganostic.

No functionality change intended.