[DebugInfo] LiveDebugValues: correctly discriminate kinds of variable locations

The missing line added by this patch ensures that only spilt variable
locations are candidates for being restored from the stack. Otherwise,
register or constant-value information can be interpreted as a spill
location, through a union.

The added regression test replicates a scenario where this occurs: the
stack load from [rsp] causes the register-location DBG_VALUE to be
"restored" to rsi, when it should be left alone. See PR43058 for details.

Un x-fail a test that was suffering from this from a previous patch.

Differential Revision: https://reviews.llvm.org/D66895

llvm-svn: 370648