[mlir][rocdl] Fixing breakage of dim operator from 904f91db

Summary:
* Update the unit test dimOp index to be an operand
* Refactored the constant naming

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

Tags: #mlir

Differential Revision: https://reviews.llvm.org/D81663