Make Value::isDereferenceablePointer handle offsets to pointer types with dereferenceable attributes

When we have a parameter (or call site return) with a dereferenceable
attribute, it can specify the size of an array pointed to by that parameter. If
we have a value for which we can accumulate a constant offset to such a
parameter, then we can use that offset in a direct comparison with the size
specified by the dereferenceable attribute.

This enables us to handle cases like this:

  int foo(int a[static 3]) {
    return a[2]; /* this is always dereferenceable */
  }

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213447 91177308-0d34-0410-b5e6-96231b3b80d8
MergedLoadStoreMotion.cpp: Fix msc17 build. Member initializer is unavailable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213448 91177308-0d34-0410-b5e6-96231b3b80d8
Handle AddrSpaceCast in stripAndAccumulateInBoundsConstantOffsets

All of the other similar functions in that part of the file look through
addrspacecast in addition to bitcast, and I see no reason why
stripAndAccumulateInBoundsConstantOffsets shouldn't do so also.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213449 91177308-0d34-0410-b5e6-96231b3b80d8