[NFCI][CostModel] Unify getCmpSelInstrCost

Add cases for icmp, fcmp and select into the switch statement of the
generic getUserCost implementation with getInstructionThroughput then
calling into it. The BasicTTI and backend implementations have be set
to return a default value (1) when a cost other than throughput is
being queried.

Differential Revision: https://reviews.llvm.org/D80550