[SelectionDAG] Transfer DbgValues when casts are optimized in SelectionDAG::getNode

Summary:
getNode optimizes (ext (trunc x)) to x and the dbgvalue node on trunc is lost. The fix calls transferDbgValues to add the dbgvalue to x.

Add DebugInfo/AArch64/dbg-value-i16.ll

Patch by Sejong Oh!

Reviewers: aprantl, javed.absar, llvm-commits, vsk

Reviewed By: aprantl, vsk

Subscribers: kristof.beyls, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D46348

llvm-svn: 331665