[DEBUGINFO] Reposting r352642: Handle restore instructions in LiveDebugValues

[DEBUGINFO] Reposting r352642: 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.

This patch was posted previously with r352642 and reverted in r352666 due
to buildbot errors. A missing return statement was the cause for the
failures.

Reviewers: aprantl, NicolaPrica

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353089 91177308-0d34-0410-b5e6-96231b3b80d8