[PowerPC] Don't assert on SELECT_CC with i1 type

When we try to select a SELECT_CC on Power9, we check if it can be matched to a
SETB instruction. In that function, we assert that the output type is i32/i64.
This is unnecessary as it is perfectly reasonable to have an i1 SELECT_CC.
Change that from an assert to an early exit condition.
Fixes: https://bugs.llvm.org/show_bug.cgi?id=45448