[SCEV] Use mustprogress flag on loops (in addition to function attribute)

This addresses a performance regression reported against 3c6e4191.  That change (correctly) limited a transform based on assumed finiteness to mustprogress loops, but the previous change (38540d71) which introduced the mustprogress check utility only handled function attributes, not the loop metadata form.

It turns out that clang uses the function attribute form for C++, and the loop metadata form for C.  As a result, 3c6e4191 ended up being a large regression in practice for C code as loops weren't being considered mustprogress despite the language semantics.