[mlir][Vector] Fold selects of single-element i1 vectors

This patch adds a folding to select operation between an all-true and all-false vector.
For now, only single element vectors (i.e., vector<1xi1>) are supported. Multi-element
cases are caught by InstCombine.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D154682