[Kernel] Minor simplification in unroll_loops; replaces a fatal by a warning.

src/kernel_internals/typing/unroll_loops.ml

tests/syntax/oracle/very_large_integers.0.res.oracle

tests/syntax/oracle/very_large_integers.1.res.oracle

tests/syntax/oracle/very_large_integers.2.res.oracle

tests/syntax/oracle/very_large_integers.3.res.oracle

tests/syntax/oracle/very_large_integers.4.res.oracle

tests/syntax/oracle/very_large_integers.5.res.oracle

tests/syntax/oracle/very_large_integers.6.res.oracle

tests/syntax/oracle/very_large_integers.7.res.oracle

tests/syntax/oracle/very_large_integers.8.res.oracle

tests/syntax/very_large_integers.c