Add brilir

[Mond45]

Adding an MLIR dialect for Bril to bril monorepo.
More info on this PR.
Link to the original repo

[sampsyo]

Really awesome work here, @Mond45. Your README was super clear; I was able to follow it easily and put the tools through their paces. Putting it all together, here's the command I ran to get all the way to ARM assembly:

bril2json < test/interp/core/tiny.bril | \
  python examples/to_ssa.py | \
  python examples/tdce.py tdce+ | \
  ./brilir/build/bin/bril2mlir 2>&1 | \
  ./brilir/build/bin/bril-opt --pass-pipeline="builtin.module(convert-bril-to-std,rename-main-function,convert-arith-to-llvm,convert-func-to-llvm,convert-cf-to-llvm,canonicalize,cse)" - | \
  $LLVM_DIR/bin/mlir-translate --mlir-to-llvmir | \
  $LLVM_DIR/bin/llc

…where $LLVM_DIR points to the freshly-built build directory from the llvm-project you have us check out at the beginning of the README. Superb!

I tried to simplify and tweak the instructions a tiny bit to be slightly more generic; hopefully that didn't mess things up too much.