I found this really weird WWM-related case whereby through the WWM transformations our isel lowering was trying to promote 2 min's into a min3 for the i8 type, which our hardware doesn't support.
The new min3_i8.ll test case would previously spew the error:
PromoteIntegerResult #0: t69: i8 = SMIN3 t70, Constant:i8<0>, t68
Before the simple fix to our isel lowering to not do it for i8 MVT's.