[MLIR] Add std.assume_alignment op.

Reviewers: ftynse, nicolasvasilache, andydavis1

Subscribers: bixia, sanjoy.google, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, Joonsoo, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74378