Make bms_prev_member work correctly with a 64 bit bitmapword

5c067521 erroneously had coded bms_prev_member assuming that a bitmapword
would always hold 32 bits and started it's search on what it thought was the
highest 8-bits of the word.  This was not the case if bitmapwords were 64
bits.

In passing add a test to exercise this function a little. Previously there was
no coverage at all.

David Rowly

src/backend/nodes/bitmapset.c

@@ -1167,7 +1167,7 @@ bms_prev_member(const Bitmapset *a, int prevbit)
 		if (w != 0)
 		{
 			int			result;
-			int			shift = 24;
+			int			shift = BITS_PER_BITMAPWORD - 8;
 			result = wordnum * BITS_PER_BITMAPWORD;
 
 			while ((w >> shift) == 0)