[mlir] [VectorOps] Implement vector.constant_mask lowering to LLVM IR

Summary:
Makes this operation runnable on CPU by generating MLIR instructions
that are eventually folded into an LLVM IR constant for the mask.

Reviewers: nicolasvasilache, ftynse, reidtatge, bkramer, andydavis1

Reviewed By: nicolasvasilache, ftynse, andydavis1

Subscribers: mehdi_amini, rriddle, jpienaar, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, stephenneuendorffer, Joonsoo, grosul1, frgossen, Kayjukh, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79815