Add a new action ('-unison') to llvm-tblgen that generates YAML code
with a description of each instruction in the specified target
(operands, itinerary, side-effects, etc.). The YAML code can be in its
turn converted into Haskell code using Unison's 'specsgen' tool. The
generated Haskell code is finally compiled and linked together with
Unison.

Extend 'MIRPrinter' and 'MIRParser' to support Unison-style MIR when
using the '-unison-mir' flag. The Unison style extends the MIR
representation with additional information needed by Unison, including
basic block frequencies, function live-out registers, memory alias
information, and simplified successor probabilities. When using the
Unison-style, the 'MIRPrintingPass' requires now a preparation pass
('UnisonMIRPrepare') that computes execution frequencies and alias
information.

Do not attach debug information to pseudo-instructions inserted by the
Unison MIR preparation pass that are used exclusively to forward
frequency information to Unison. This prevents a crash in the pass
when empty basic blocks are handled.

Use 64-bits ints to to avoid overflows when storing the fixpoint basic
block frequencies from the 'BlockFrequency' class. Update the type in
the corresponding metadata node emitted in the Unison-style MIR.
Update the unison repo so that ints of different widths are accepted
in metadata nodes.

Add script to run 'llc' with Unison. The script runs llc twice
to generate Unison's input, then Unison itself, and then llc
again to emit the generated assembly code. It has the same
interface as llc itself, plus a few additional flags to control
Unison.