Fix a debug location handling bug in GVN.

Caught by the famous "DebugLoc describes the currect SubProgram" assertion.

When GVN is removing a nonlocal load it updates the debug location of the
SSA value it replaced the load with with the one of the load. In the
testcase this actually overwrites a valid debug location with an empty one.

In reality GVN has to make an arbitrary choice between two equally valid
debug locations. This patch changes to behavior to only update the
location if the value doesn't already have a debug location.

llvm-svn: 245588