Remove underflow error in float division with infinite divisor.

float4_div and float8_div correctly produced zero for zero divided
by infinity, but threw an underflow error for nonzero finite values
divided by infinity.  This seems wrong; at the very least it's
inconsistent with the behavior recently implemented for numeric
infinities.  Remove the error and allow zero to be returned.

This patch also removes a useless isinf() test from the overflow
checks in these functions (non-Inf divided by Inf can't produce Inf).

Extracted from a larger patch; this seems significant outside the
context of geometric operators, so it deserves its own commit.

Kyotaro Horiguchi

Discussion: https://postgr.es/m/CAGf+fX70rWFOk5cd00uMfa__0yP+vtQg5ck7c2Onb-Yczp0URA@mail.gmail.com

src/test/regress/sql/float8.sql

@@ -42,6 +42,7 @@ SELECT ' INFINITY    x'::float8;
 
 SELECT 'Infinity'::float8 + 100.0;
 SELECT 'Infinity'::float8 / 'Infinity'::float8;
+SELECT '42'::float8 / 'Infinity'::float8;
 SELECT 'nan'::float8 / 'nan'::float8;
 SELECT 'nan'::float8 / '0'::float8;
 SELECT 'nan'::numeric::float8;