An operator new with an empty exception specifier returns null on a bad

An operator new with an empty exception specifier returns null on a bad
allocation and therefore requires a null-check.  We were doing that, but
we weren't treating the new-initializer as being conditionally executed,
which means it was possible to get ill-formed IR as in PR9298.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@127147 91177308-0d34-0410-b5e6-96231b3b80d8