[DEBUGINFO] Handle restore instructions in LiveDebugValues

The LiveDebugValues pass recognizes spills but not restores, which can 
cause large gaps in location information for some variables, depending
on control flow. This patch make LiveDebugValues recognize restores and
generate appropriate DBG_VALUE instructions. 

Reviewers: aprantl, NicolaPrica

Differential Revision: https://reviews.llvm.org/D57271

llvm-svn: 352642