[SCEV] Track no-abnormal-exits instead of no-throw calls

[SCEV] Track no-abnormal-exits instead of no-throw calls

Absence of may-unwind calls is not enough to guarantee that a
UB-generating use of an add-rec poison in the loop latch will actually
cause UB.  We also need to guard against calls that terminate the thread
or infinite loop themselves.

This partially addresses PR28012.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272181 91177308-0d34-0410-b5e6-96231b3b80d8