DAGCombine: always constant fold FMA when target disable FP exceptions

Summary: When trying to constant fold an FMA in the DAG, getNode()
fails to fold the FMA if an operand is not finite. In this case this
patch allows the constant folding if !TLI->hasFloatingPointExceptions()

Reviewers: resistor

Reviewed By: resistor

Subscribers: hfinkel, llvm-commits

Differential Revision: http://reviews.llvm.org/D6912

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 226901