[IR] Add the constructor of ShuffleVector for one-input-vector.

One of the two inputs of the Shufflevector is often a placeholder.
Previously, there were cases where the placeholder was undef, and there were cases where it was poison.
I added these constructors to create a placeholder consistently.

Changing to use the newly added constructor will be written in a separate patch.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D110146