[mlir] [VectorOps] Replace zero-scalar + splat into direct zero vector constant

Summary:
The scalar zero + splat yields more intermediate code than the direct
dense zero constant, and ultimately is lowered to exactly the same
LLVM IR operations, so no point wasting the intermediate code.

Reviewers: nicolasvasilache, andydavis1, reidtatge

Reviewed By: nicolasvasilache

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/D79758