When emitting a multidimensional array new, emit the initializers for the

When emitting a multidimensional array new, emit the initializers for the
trailing elements as a single loop, rather than sometimes emitting a nest of
several loops. This fixes a bug where CodeGen would sometimes try to emit an
expression with the wrong type for the element being initialized. Plus various
other minor cleanups to the IR produced for array new initialization.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210079 91177308-0d34-0410-b5e6-96231b3b80d8