bitint: fix inconsistency in bitint_precision_kind

bitint: fix inconsistency in bitint_precision_kind

This patch fixes an issue where the first call to bitint_precision_kind would
return the bitint kind, which was also inconsistent with subsequent calls with
the same bitint size.

gcc/ChangeLog:

	* gimple-lower-bitint.cc (bitint_precision_kind): Fix inconsistency in
	results between first and consecutive calls to this function.