[bugpoint] Fix "Alias must point to a definition" problems
GlobalAliases may reference function definitions, but not function declarations.
bugpoint would sometimes create invalid IR by deleting a function's body (thus
mutating a function definition into a declaration) without first 'fixing' any
GlobalAliases that reference that function definition.
This change iteratively prevents that issue. Before deleting a function's body,
it scans the module for GlobalAliases which reference that function. When
found, it eliminates them using replaceAllUsesWith.
Fixes PR20788.
Patch by Nick Johnson!
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254171
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