Allow more top-down inlining for single-BB callees

This means that things like `<usize as Step>::forward_unchecked` and `<PartialOrd for f32>::le` will inline even if we've already done a bunch of inlining to find the calls to them.

Author: scottmcm

tests/fail/both_borrows/aliasing_mut4.tree.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: write access through <TAG> at ALLOC[0x0] is forbidden
   --> RUSTLIB/core/src/mem/mod.rs:LL:CC
    |
-LL |         ptr::write(dest, src);
-   |         ^^^^^^^^^^^^^^^^^^^^^ write access through <TAG> at ALLOC[0x0] is forbidden
+LL |         crate::intrinsics::write_via_move(dest, src);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ write access through <TAG> at ALLOC[0x0] is forbidden
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
    = help: the accessed tag <TAG> is foreign to the protected tag <TAG> (i.e., it is not a child)