Simplify regression in #560

[dsharlet]

#560 has a change reverted by the following diff:

diff --git a/builder/simplify.cc b/builder/simplify.cc
index aeaa156..84e9555 100644
--- a/builder/simplify.cc
+++ b/builder/simplify.cc
@@ -1239,7 +1239,7 @@ public:
     if (auto cstep = as_constant(step)) alignment.modulus = *cstep;
     if (auto cmin = as_constant(bounds.min)) alignment.remainder = *cmin;
     // If we're in the body of the loop, then we know that bounds.max >= bounds.min.
-    stmt body = mutate_with_bounds(op->body, op->sym, {bounds.min, mutate(max(bounds.min, bounds.max))}, alignment);
+    stmt body = mutate_with_bounds(op->body, op->sym, {bounds.min, bounds.max}, alignment);
     for (auto& i : buffers) {
       if (i) --i->loop_depth;
     }

This change slightly regressed the simplification of this example:

SLINKY_VERBOSE=1 bazel run --config=ubsan builder/test:pipeline -- --gtest_filter=split_schedule_mode/elementwise.pipeline_2d/2147483647_1_0

from producing:

out.y = loop(parallel, [buffer_min(out, 1), buffer_max(out, 1)], 1) {
 closure {in, out, out.y} in {
  out.out.y = crop_dim(out, 1, [out.y, out.y]) {
   intm.out.y = crop_dim(out, 1, [out.y, out.y]) {
    out.x = loop(parallel, [buffer_min(out, 0), buffer_max(out, 0)], 1) {
     closure {in, out, out.x, out.out.y, intm.out.y} in {
      intm.out.x = crop_dim(intm.out.y, 0, [out.x, out.x]) {
       call(<anonymous target>, {in}, {intm.out.x})
      }
      out.out.x = crop_dim(out.out.y, 0, [out.x, out.x]) {
       call(<anonymous target>, {out}, {out.out.x})
      }
     }
    }
   }
  }
 }
}

vs.

out.y = loop(parallel, [buffer_min(out, 1), buffer_max(out, 1)], 1) {
 closure {in, out, out.y} in {
  out.out.y = crop_dim(out, 1, [out.y, out.y]) {
   intm.out.y = crop_dim(out, 1, [out.y, out.y]) {
    out.x = loop(parallel, [buffer_min(out, 0), buffer_max(out, 0)], 1) {
     closure {in, out, out.x, out.out.y, intm.out.y} in {
      out.out.x = crop_dim(out.out.y, 0, [out.x, out.x]) {
       intm.out.x = crop_dim(intm.out.y, 0, [out.x, buffer_max(out.out.x, 0)]) {
        call(<anonymous target>, {in}, {intm.out.x})
       }
       call(<anonymous target>, {out}, {out.out.x})
      }
     }
    }
   }
  }
 }
}

Note the extra nesting of the intm.out.x = crop_dim(...).