Fix elf core file VMA-contiguous region collapsing.

Elf core files were collapsing core segments when the virtual memory
addresses were contiguous without checking if the core-file-backed
memory region was the same size as the segment's VMA region. Without
this extra check, any time regions were collapsed but the core-backed
region was smaller (and thus had a zero-filled hole at the end), the
collapse operation would break VMA to core file lookups for subsequent
collapsed regions.

This change fixes the following bug:
http://llvm.org/bugs/show_bug.cgi?id=18769



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@201214 91177308-0d34-0410-b5e6-96231b3b80d8