Remove assertions from the hot value_def() function.
Remove assertions from the hot value_def() function.

The errors caught by these assertions are also detected by the IL
verifier.

Speed up compilation by 13%.

sunfishcode/cretonne,stoklund/cretonne,sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne,stoklund/cretonne