stage1: small mem usage improvement for IR

move a boolean field to be represented implicitly with the enum tag.
Just borrowing one of the many strategies of stage2.

This simple change took the peak mem usage from std lib tests on
my machine from 8.21 GiB to 8.11 GiB.
5cbb642525Andrew Kelley committed on 3/18/2021, 8:14:26 PMparent17c066e
3 files changedLine totals unavailable