3000x speed improvement on compose-affine-maps by dropping NestedMatcher for

a trivial inst walker :-) (reduces pass time from several minutes non-terminating to 120ms) - (fixes b/123541184)

- use a simple 7-line inst walker to collect affine_apply op's instead of the nested
  matcher; -compose-affine-maps pass runs in 120ms now instead of 5 minutes + (non-
  terminating / out of memory) - on a realistic test case that is 20,000 lines 12-d
  loop nest

- this CL is also pushing for simple existing/standard patterns unless there
  is a real efficiency issue (OTOH, fixing nested matcher to address this issue requires
  cl/231400521)

- the improvement is from swapping out the nested walker as opposed to from a bug
  or anything else that this CL changes

- update stale comment

PiperOrigin-RevId: 231623619