DebugInfo: Avoid propagating incorrect debug locations in SelectionDAG via CSE.

Summary:
When SelectionDAG performs CSE it is possible that the context's source
location is different from that of the selected node. This can lead to
incorrect line number records. We update the debug location to the
one that occurs earlier in the instruction sequence.

This fixes PR21006.

Reviewers: echristo, sdmitrouk

Subscribers: jevinskie, asl, llvm-commits

Differential Revision: http://reviews.llvm.org/D12094

llvm-svn: 268323