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.

llvm-svn: 73012