In dpow(), remove redundant check for whether y is an integer.

I failed to notice that we don't really need to check for y being an
integer in the code path where x = -inf; we already did.

Also make some further cosmetic rearrangements in that spot in hopes
of dodging the seeming compiler bug that buildfarm member fossa is
hitting.  And be consistent about declaring variables as "float8"
not "double", since the pre-existing variables in this function are
like that.

Discussion: https://postgr.es/m/E1jkyFX-0005RR-1Q@gemulon.postgresql.org

src/backend/utils/adt/float.c

@@ -1546,7 +1546,7 @@ dpow(PG_FUNCTION_ARGS)
 	 */
 	if (isinf(arg2))
 	{
-		double		absx = fabs(arg1);
+		float8		absx = fabs(arg1);
 
 		if (absx == 1.0)
 			result = 1.0;
@@ -1578,16 +1578,15 @@ dpow(PG_FUNCTION_ARGS)
 		}
 		else					/* x = -Inf */
 		{
-			bool		yisoddinteger = false;
-
-			if (arg2 == floor(arg2))
-			{
-				/* y is integral; it's odd if y/2 is not integral */
-				double		halfy = arg2 / 2;	/* should be computed exactly */
+			/*
+			 * Per POSIX, the sign of the result depends on whether y is an
+			 * odd integer.  Since x < 0, we already know from the previous
+			 * domain check that y is an integer.  It is odd if y/2 is not
+			 * also an integer.
+			 */
+			float8		halfy = arg2 / 2;	/* should be computed exactly */
+			bool		yisoddinteger = (floor(halfy) != halfy);
 
-				if (halfy != floor(halfy))
-					yisoddinteger = true;
-			}
 			if (arg2 > 0.0)
 				result = yisoddinteger ? arg1 : -arg1;
 			else
@@ -1622,7 +1621,7 @@ dpow(PG_FUNCTION_ARGS)
 				result = 0.0;	/* we already verified y is positive */
 			else
 			{
-				double		absx = fabs(arg1);
+				float8		absx = fabs(arg1);
 
 				if (absx == 1.0)
 					result = 1.0;