Replace return code checks with OP_ALWAYS_TRUE().

Instead of assigning the return code to a local variable and then
 using OP_ASSERT(), define a new OP_ALWAYS_TRUE() macro that still
 evaluates its argument when assertions are disabled.
This avoids -Wunused-but-set-variable warnings from clang and
 useless scan-build reports (if scan-build is run without
 assertions enabled).