Better error messages for 'llvm_return' misuse.
Better error messages for 'llvm_return' misuse.

Most importantly, I changed

    simulator returned value when not expected

to

    simulator returned value when not expected (add an 'llvm_return' statement?)"

because the old message is not very helpful if you don't know about
`llvm_return`.

GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script