[DAGCombiner] Minor compile time improvement to (sext_in_reg...

[DAGCombiner] Minor compile time improvement to (sext_in_reg (sign_extend_vector_inreg x)) optimization.

Don't bother calling ComputeNumSignBits if N00Bits < ExtVTBits. No
matter what answer we get back this will be true:
(N00Bits - DAG.ComputeNumSignBits(N00, DemandedSrcElts)) < ExtVTBits)

So we might as well save the computation. This makes the code more
consistent with the similar (sext_in_reg (sext x)) handling above.