[BasicTTI] Remove unused support for multiple opcodes in getTypeBasedIntrinsicInstrCost [nfc]

ISDs only ever contains a single ISD opcode.  We can simplify the code under this assumption. The code being removed was added back in 2016 in 0f26b0ae to support FMAXNAN/FMINNAN, but at some point since then the motivating case was rewritten not to use the ISDs mechanism.  No reason to keep the false generality around now.