[Mem2Reg] Consistently preserve nonnull assume for uninit load

When performing a !nonnull load from uninitialized memory, we
should preserve the nonnull assume just like in all other cases.
We already do this correctly in the generic mem2reg code, but
don't handle this case when using the optimized single-block
implementation.

Make sure that the optimized implementation exhibits the same
behavior as the generic implementation.