[IndVarSimplify] Don't assert that terminator is not SCEVable (PR55925)

The IV widening code currently asserts that terminators aren't SCEVable
-- however, this is not the case for invokes with a returned attribute.

As far as I can tell, this assertions is not necessary -- even if we
have a critical edge (the second test case), the trunc gets inserted
in a legal position.

Fixes https://github.com/llvm/llvm-project/issues/55925.

Differential Revision: https://reviews.llvm.org/D127288