[MSAN] Add unary FNeg visitor to the MemorySanitizer

[cameron.mcinally]

Summary

I'm not familiar with this pass, so a thorough review is appreciated.

Note that I was not able to find existing tests for the binary FP operators. It seems as if they're just pass-throughs since they don't touch memory. Or did I miss something?

[eugenis]

The instrumentation pass change looks good.
There is a test for fdiv in msan_basic.ll, you can do something similar.

[cameron.mcinally]

Thanks, @eugenis. Not sure how I missed that..

[eugenis]

Thanks!
LGTM

llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp

#endif
   void handleShadowOr(Instruction &I) {
     IRBuilder<> IRB(&I);
     ShadowAndOriginCombiner SC(this, IRB);
     for (Instruction::op_iterator OI = I.op_begin(); OI != I.op_end(); ++OI)
       SC.Add(OI->get());
     SC.Done(&I);
   }
 
+  void visitFNeg(UnaryOperator &I) { handleShadowOr(I); }
+
   // Handle multiplication by constant.
   //
   // Handle a special case of multiplication by constant that may have one or
   // more zeros in the lower bits. This makes corresponding number of lower bits
   // of the result zero as well. We model it by shifting the other operand
   // shadow left by the required number of bits. Effectively, we transform
   // (X * (A * 2**B)) to ((X << B) * A) and instrument (X << B) as (Sx << B).
   // We use multiplication by 2**N instead of shift to cover the case of