A def. operand of a machine instruction may be an ordinary Value*,

A def. operand of a machine instruction may be an ordinary Value*,
not just an Instruction*, at least in one unfortunate case:
the first operand to the va_arg instruction.
Modify ValueToDefVecMap to map from Value*, not Instruction*.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@7052 91177308-0d34-0410-b5e6-96231b3b80d8