[MachineVerifier] Avoid reference to nullptr
[MachineVerifier] Avoid reference to nullptr

Instantiation of the MachineVerifierPass through
PassInfo::getNormalCtor would yield a segfault since the default
constructor of the MachineVerifierPass takes a reference to nullptr.

Patch by Simone Pellegrini.

Differential Revision: https://reviews.llvm.org/D31387



git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@298987 91177308-0d34-0410-b5e6-96231b3b80d8

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