Slightly generalize the code that handles shuffles of consecutive loads

Slightly generalize the code that handles shuffles of consecutive loads
on x86 to handle more cases.  Fix a bug in said code that would cause it
to read past the end of an object.  Rewrite the code in
SelectionDAGLegalize::ExpandBUILD_VECTOR to be a bit more general.
Remove PerformBuildVectorCombine, which is no longer necessary with
these changes.  In addition to simplifying the code, with this change,
we can now catch a few more cases of consecutive loads.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73012 91177308-0d34-0410-b5e6-96231b3b80d8