iropt-test: Run tests with and without constant folding.

iropt-test: Run tests with and without constant folding.

That way we can check whether the result of constant folding matches
the value computed by the insn sequence that is generated to implement
a specific IROp. Bugs were found for all architectures. s390 has been
fixed. The other architectures are currently disabled.

- irops.tab modified to enable only those IROps that are actually
  implemented for a given architecture. Architectures considered here
  are amd64, x86, ppc32, ppc64be/le
- IRICB_iropt_payload amended to store both the folded and unfolded
  result
- In valgrind_execute_test call IR injection twice: with and without
  constant folding enabled (--vex-iropt-fold-expr=yes/no)