revert r243687: enable fast-math-flag propagation to DAG nodes

We can't propagate FMF partially without breaking DAG-level CSE. We either need to
relax CSE to account for mismatched FMF as a temporary work-around or fully propagate
FMF throughout the DAG.

Surprisingly, there are no existing regression tests for this, but here's an example:

  define float @fmf(float %a, float %b) {
    %mul1 = fmul fast float %a, %b
    %nega = fsub fast float 0.0, %a
    %mul2 = fmul fast float %nega, %b
    %abx2 = fsub fast float %mul1, %mul2
    ret float %abx2
  }


$ llc -o - badflags.ll -march=x86-64 -mattr=fma -enable-unsafe-fp-math -enable-fmf-dag=0
...
    vmulss    %xmm1, %xmm0, %xmm0
    vaddss    %xmm0, %xmm0, %xmm0
    retq

$ llc -o - badflags.ll -march=x86-64 -mattr=fma -enable-unsafe-fp-math -enable-fmf-dag=1
...
    vmulss    %xmm1, %xmm0, %xmm2
    vfmadd213ss    %xmm2, %xmm1, %xmm0  <--- failed to recognize that (a * b) was already calculated
    retq

llvm-svn: 244053

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -79,7 +79,7 @@ LimitFPPrecision("limit-float-precision",
                  cl::init(0));
 
 static cl::opt<bool>
-EnableFMFInDAG("enable-fmf-dag", cl::init(true), cl::Hidden,
+EnableFMFInDAG("enable-fmf-dag", cl::init(false), cl::Hidden,
                 cl::desc("Enable fast-math-flags for DAG nodes"));
 
 // Limit the width of DAG chains. This is important in general to prevent