[DAG] Rewrite areNonVolatileConsecutiveLoads to use BaseIndexOffset

As discussed in D34087, rewrite areNonVolatileConsecutiveLoads using
generic checks. Also, propagate missing local handling from there to
BaseIndexOffset checks.

Tests of note:

  * test/CodeGen/X86/build-vector* - Improved.
  * test/CodeGen/BPF/undef.ll - Improved store alignment allows an
    additional store merge

  * test/CodeGen/X86/clear_upper_vector_element_bits.ll - This is a
    case we already do not handle well. Here, the DAG is improved, but
    scheduling causes a code size degradation.

Reviewers: RKSimon, craig.topper, spatel, andreadb, filcab

Subscribers: nemanjai, llvm-commits

Differential Revision: https://reviews.llvm.org/D34472

llvm-svn: 306819