JIT: clear up some no-op casts in the importer (#17996)

JIT: clear up some no-op casts in the importer (#17996)

If we see a no-op cast (say converting long to ulong) where the operand
to be cast already has the right stack type, don't generate a GT_CAST,
just use the operand.

This comes up in some cases with unsafe operations where the pointer type
is cast. Interposing a cast between an GT_IND and GT_ADDR results in poor
codegen as the jit thinks the addressed local is address taken.