IVDescriptors: clarify getSCEV use in a function (NFC) (#106222)
getSCEV will assert unless the operand is SCEVable. Replace an instance
of the implementation of ScalarEvolution::isSCEVable (which checks that
the operand is either integer or pointer type) with a call to the
function, to make it clear that the subsequent use of getSCEV will not
fail.