[MLIR] Added llvm.fence

This patch adds llvm.fence. I tried not to change the syntax much.

syntax:

LLVM IR
`fence [syncscope("<target-scope>")] <ordering>`

MLIR LLVM Dialect

`llvm.fence [syncscope("<target-scope>")] <ordering>`

example:
LLVM IR: `fence syncscope("agent") seq_cst`
MLIR: `llvm.fence syncscope("agent") seq_cst`

Differential Revision: https://reviews.llvm.org/D75645