[X86] Fix the inversion of low and high bits for the lowering of MUL_LOHI.
Also add a few comments.

<rdar://problem/17581756>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212808 91177308-0d34-0410-b5e6-96231b3b80d8