AsmPrinter: Use emitGlobalConstantFP to emit elements of constant data

It's strange to duplicate the logic for emitting FP values into
emitGlobalConstantDataSequential, and it's even stranger that we end
up printing the verbose assembly comments differently between the two
paths. Just call into emitGlobalConstantFP rather than crudely
duplicating its logic.

llvm-svn: 254988