Improve performance of float overflow checks in btree_gist

Improve performance of float overflow checks in btree_gist

The current code could do unnecessary calls to isinf() (two for the
argument values all the time while one could be sufficient in some
cases).  zero_is_valid was never used but the result value was still
checked on 0 in the first position of the check.

This is similar to 607f8ce.  btree_gist has just copy-pasted the code
doing those checks from the backend float4/8 code, as of the macro
CHECKFLOATVAL(), to do the work.

Author: Haiying Tang
Discussion: https://postgr.es/m/OS0PR01MB611358E3A7BC3C2F874AC36BFBF39@OS0PR01MB6113.jpnprd01.prod.outlook.com