[docs] Reword documentation in terms of SCCs not cycles
Given the example:
br i1 %c, label %next, label %header
br i1 %c2, label %exit, label %header
We end up with a loop containing both header and next. Given that, the describing the loop in terms of cycles is confusing since we have multiple distinct cycles within a single Loop. Standardize on the SCC to clarify.
Differential Revision: https://reviews.llvm.org/D65299