Add combine functions for various floating-point aggregates.

Add combine functions for various floating-point aggregates.

This allows parallel aggregation to use them.  It may seem surprising
that we use float8_combine for both float4_accum and float8_accum
transition functions, but that's because those functions differ only
in the type of the non-transition-state argument.

Haribabu Kommi, reviewed by David Rowley and Tomas Vondra