Avoid undefined behavior in LinkAllPasses.h

The LinkAllPasses.h file is included in several main programs, to force
a large number of passes to be linked in.  However, the ForcePassLinking
constructor uses undefined behavior, since it calls member functions on
`nullptr`, e.g.:

      ((llvm::Function*)nullptr)->viewCFGOnly();
      llvm::RGPassManager RGM;
      ((llvm::RegionPass*)nullptr)->runOnRegion((llvm::Region*)nullptr, RGM);

When the optimization level is -O2 or higher, the code below the first
nullptr dereference is optimized away, and replaced by `ud2` (on x86).

Therefore, the calls after that first dereference are never emitted.  In
my case, I noticed there was no call to `llvm::sys::RunningOnValgrind()`!

Replace instances of dereferencing `nullptr` with either objects on the
stack, or regular function calls.

Differential Revision: http://reviews.llvm.org/D15996

llvm-svn: 257645

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