[InstCombine] Run the cast.ll test a twice, now also testing little endian. NFC

Some tests in test/Transforms/InstCombine/cast.ll depend on
endianness. Added a second run line to run the tests with both
big and little endian. In the past we only compiled for big
endian, and then it was hard to see if any big endian bugfixes
would impact the little endian result etc.