[mlir] Introduce bare ptr calling convention for MemRefs in LLVM dialect

Summary:
This patch introduces an alternative calling convention for
MemRef function arguments in LLVM dialect. It converts MemRef
function arguments to LLVM bare pointers to the MemRef element
type instead of creating a MemRef descriptor. Bare pointers are
then promoted to a MemRef descriptors at the beginning of the
function. This calling convention is only enabled with a flag.

Reviewers: ftynse, bondhugula, nicolasvasilache, rriddle, mehdi_amini

Reviewed By: ftynse, rriddle, mehdi_amini

Subscribers: Joonsoo, flaub, merge_guards_bot, jholewinski, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, csigg, arpith-jacob, mgester, lucyrfox, herhut, aartbik, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72802