[TableGen] Introduce !listsplat 'binary' operator

Summary:
```
``!listsplat(a, size)``
    A list value that contains the value ``a`` ``size`` times.
    Example: ``!listsplat(0, 2)`` results in ``[0, 0]``.
```

I plan to use this in X86ScheduleBdVer2.td for LoadRes handling.

This is a little bit controversial because unlike every other binary operator
the types aren't identical.

Reviewers: stoklund, javed.absar, nhaehnle, craig.topper

Reviewed By: javed.absar

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60367

llvm-svn: 358117