[mlir] [VectorOps] Improve vector.constant_mask lowering

Use direct vector constants for the 1-D case. This approach
scales much better than generating elaborate insertion operations
that are eventually folded into a constant. We could of course
generalize the 1-D case to higher ranks, but this simplification
already helps in scaling some microbenchmarks that would formerly
crash on the intermediate IR length.

Reviewed By: reidtatge

Differential Revision: https://reviews.llvm.org/D82144