Fix TableGen so that it will emit the correct signature for FastEmit_f:

  /// FastEmit_f - This method is called by target-independent code
  /// to request that an instruction with the given type, opcode, and
  /// floating-point immediate operand be emitted.
  virtual unsigned FastEmit_f(MVT VT,
                              MVT RetVT,
                              unsigned Opcode,
                              const ConstantFP *FPImm);

Currently, it emits an accidentally overloaded version without the const on the
ConstantFP*. This doesn't affect anything in the tree, since nothing causes that
method to be autogenerated, but I have been playing with some ARM TableGen
refactorings that hit this problem.

llvm-svn: 147727