DebugInfo: Track spilled variables in LiveDebugValues

When variables are spilled to the stack by the register allocator, keep track of their
debug locations in LiveDebugValues and insert DBG_VALUE instructions at the appropriate
place. Ensure that the locations are propagated down the dominator tree via the existing 
mechanisms.

Reviewer: aprantl

Differential Revision: https://reviews.llvm.org/D29500

llvm-svn: 294356