Bug 7440: Add an ins--iterate convenience macro to emit loops

* sources/lib/llvm/llvm-builder.dylan
  (ins--iterate): New convenience macro for emitting looping constructs
   as LLVM IR.
  (ins--iterate-aux): New auxiliary macro definition.
  (do-add-iterate-phi-operand): New auxiliary function for adding
   operands to loop head phi nodes.

* sources/lib/llvm/llvm-library.dylan
  (module llvm-builder): Export ins--iterate.

* sources/lib/llvm/tests/specification.dylan
  (module-spec llvm-builder): Add macro-test ins--iterate-test.

* sources/lib/llvm/tests/llvm-builder-test.dylan
  (macro-test ins--iterate-test): New tests for ins--iterate.
Bug 7458: Snapshot