Fix a variant of PR6112 found by thinking about it: when doing

RAUW of a global variable with a local variable in function F,
if function local metadata M in function G was using the global
then M would become function-local to both F and G, which is not
allowed.  See the testcase for an example.  Fixed by detecting
this situation and zapping the metadata operand when it occurs.

llvm-svn: 103007