possible use-after-free in codegen

[JeffBezanson]

From valgrind:

==14927== Invalid read of size 8
==14927==    at 0x4F0151F: llvm::Use::getNext() const (Use.h:108)
==14927==    by 0x4F69F60: llvm::Value::use_iterator_impl<llvm::Use>::operator++() (Value.h:134)
==14927==    by 0x4F52903: emit_function(_jl_method_instance_t*, _jl_code_info_t*, unsigned long, _jl_llvm_functions_t*, jl_cgparams_t const*) (codegen.cpp:5846)
==14927==    by 0x4F1DAB7: jl_compile_linfo (codegen.cpp:1153)
==14927==    by 0x4F1E6BA: jl_generate_fptr (codegen.cpp:1366)
==14927==    by 0x4E6C89B: jl_compile_method_internal (julia_internal.h:357)
==14927==    by 0x4E6CA0F: jl_call_method_internal (julia_internal.h:385)
==14927==    by 0x4E73116: jl_apply_generic (gf.c:1929)
==14927==    by 0x40555F4: ??? (in /run/shm/julia-codegen-14927-846930886 (deleted))
==14927==    by 0x4E79F29: jl_call_fptr_internal (julia_internal.h:370)
==14927==    by 0x4E7A03A: jl_call_method_internal (julia_internal.h:389)
==14927==    by 0x4E7A944: fl_invoke_julia_macro (ast.c:220)
==14927==  Address 0xf9654e8 is 8 bytes inside a block of size 88 free'd
==14927==    at 0x4C2C2BC: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==14927==    by 0x66659C0: llvm::Instruction::eraseFromParent() (in /home/jeff/src/julia/usr/lib/libLLVM-3.9.so)
==14927==    by 0x4F528F4: emit_function(_jl_method_instance_t*, _jl_code_info_t*, unsigned long, _jl_llvm_functions_t*, jl_cgparams_t const*) (codegen.cpp:5848)
==14927==    by 0x4F1DAB7: jl_compile_linfo (codegen.cpp:1153)
==14927==    by 0x4F1E6BA: jl_generate_fptr (codegen.cpp:1366)
==14927==    by 0x4E6C89B: jl_compile_method_internal (julia_internal.h:357)