[DebugInfo] Handle endianness when moving debug info for split integer values (reapplied)

Summary:
Take the target's endianness into account when splitting the
debug information in DAGTypeLegalizer::SetExpandedInteger.

This patch fixes so that, for big-endian targets, the fragment
expression corresponding to the high part of a split integer
value is placed at offset 0, in order to correctly represent
the memory address order.

I have attached a PPC32 reproducer where the resulting DWARF
pieces for a 64-bit integer were incorrectly reversed.

Original patch was reverted due to using -stop-after=isel in
the test case (but that is only working when AMDGPU target
is included in the llc build). The test case has now been
updated to use -stop-before=expand-isel-pseudos instead.

Patch by: dstenb

Reviewers: JDevlieghere, aprantl, dblaikie

Reviewed By: JDevlieghere, aprantl, dblaikie

Subscribers: nemanjai

Differential Revision: https://reviews.llvm.org/D38172

llvm-svn: 314781