lowering / codegen is unsound for try/catch variables

[vtjnash]

The LLVM optimizer can be tricked into not observing memory effects on will-throw branches, since it does not know they will resume at the catch branch. We can turn that into a miscompile using SSAValue (as otherwise slot2ssa would normally put a phic node here on any attempted direct use of x which would inhibit llvm from doing any analysis of it)

julia> function foo()
       x = Ref(0)
       for i in ((0,x), (1,x))
       i[1] == 1 && return i[2][]
       try; i[2][] = 1; throw("intentional"); catch; end
       end
       throw("unreachable")
       end
foo (generic function with 1 methods)

julia> foo() # this should return 1
0