[FunctionAttrs] Identify norecurse functions

A function can be marked as norecurse if:
  * The SCC to which it belongs has cardinality 1; and either
    a) It does not call any non-norecurse function. This includes self-recursion; or
    b) It only has one callsite and the function that callsite is within is marked norecurse.

a) is best propagated bottom-up and b) is best propagated top-down.

We build up the norecurse attributes bottom-up using the existing SCC pass, and mark functions with no obvious recursion (but not provably norecurse) to sweep later, top-down.

llvm-svn: 252862