[SelectionDAG] Make the code in mutateStrictFPToFP less aware of how many...

[SelectionDAG] Make the code in mutateStrictFPToFP less aware of how many operands each node has. NFCI

Just copy all of the operands except the chain and call MorphNode on that.
This removes the IsUnary and IsTernary flags.

Also always get the result type from the result type of the original
nodes. Previously we got it from the operand except for two nodes
where that didn't work.

llvm-svn: 362269