[RISCV] Remove overly restrictive assert from negateFMAOpcode.

It's possible that both multiplicands are being negated. This won't
change the opcode, but we can delete the two negates. Allow this
case to get through negateFMAOpcode.

I think D152260 will also fix this test case, but in the future
it may be possible for an fneg to appear after we've already converted
to RISCVISD opcodes in which case D152260 won't help.

Reviewed By: fakepaper56

Differential Revision: https://reviews.llvm.org/D152296