[X86] Make sure we don't read too many operands from X86ISD::FMADDS1/FMADDS3...

[X86] Make sure we don't read too many operands from X86ISD::FMADDS1/FMADDS3 nodes when doing FNEG combine.

r317453 added new ISD nodes without rounding modes that were added to an existing if/else chain. But all the previous nodes handled there included a rounding mode. The final code after this if/else chain expected an extra operand that isn't present for the new nodes.

llvm-svn: 317748