Temporarily disable return type assertion to reduce noise in testing

Disable the following assertion in parse1.cpp temporarily, to reduce noise in the testing pipelines while the root cause of its failures is investigated in JDK-8305185:

assert(false) failed: Can't determine return type.

This assertion was added by JDK-8303951 to force the re-evaluation of compilation bailouts. The invariant violation it catches is harmless (other than potentially causing a performance degradation) because it leads to a compilation bailout.