[RGT] Recode more unreachable assertions and tautologies

[RGT] Recode more unreachable assertions and tautologies

Count iterations of zero-trip loops and assert the count is zero,
rather than asserting inside the loop.
Unreachable functions should use llvm_unreachable.
Remove tautological 'if' statements, even when they're following a
pattern of checks.

Found by the Rotten Green Tests project.