Try/catch handling

[MikeInnes]

Representing try/catch in SSA form, and converting it back to the usual lowered-code form for code generation.

Previously IRTools had no notion of try/catch at all. I never tested this but presumably it blew up badly in those cases. Now try/catch works great with dynamos and tools like Zygote can handle it as they wish.

Example usage with interesting dataflow:

julia> function foo()
         x = 1
         y = 1
         try
           y = bar()
         catch e
           x, y
         end
         x, y
       end
foo (generic function with 2 methods)

julia> @code_ir foo()
1: (%1)
  %2 = try #2
  %3 = catch 2 (1, 1)
  %4 = Main.bar()
  %5 = catch 2 (1, %4)
  %6 = end try
  br 3 (1, %4)
2: (%7, %8)
  %9 = end try
  %10 = $(Expr(:the_exception))
  %11 = Core.tuple(%7, %8)
  %12 = pop exception %2
  br 3 (%7, %8)
3: (%13, %14)
  %15 = Core.tuple(%13, %14)
  return %15

try and end try correspond to the usual enter and leave. catch is a pseudo-branch that says "if an error is thrown after this point, branch to #2 with the following arguments". Those argument can change over the course of the try block, so multiple catch branches are allowed.

[aviatesk]

@MikeInnes thanks for addressing this !