[DebugInfo] Test for variable range un-coalescing

LiveDebugVariables can coalesce ranges of variable locations across
multiple basic blocks. However when it recreates DBG_VALUE instructions,
it has to recreate one DBG_VALUE per block, otherwise it doesn't
represent the pre-regalloc layout and variable assignments can go missing.

This feature works -- however while mucking around with LiveDebugVariables,
I commented the relevant code it out and no tests failed. Thus, here's a
test that checks LiveDebugVariables preserves DBG_VALUEs across block
boundaries.

Differential Revision: https://reviews.llvm.org/D66347

llvm-svn: 369243