Fix a bug that mergeCases table does not match the merge constants.

MergeCases table should not have an entry for MergeContents because atoms with
MergeContents attribute should never have name. This issue was not caught by a
test because getting a value of 6th element of an array of array actually gets
the first element's value of the next array, and that happened to be a valid
value. Added asserts to catch that error.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@203322 91177308-0d34-0410-b5e6-96231b3b80d8