Improve the behavior of control flow operations (``dr.if_stmt(), drjit.while_loop()``) in AD-suspended scopes

[wjakob]

Dr.Jit control flow operations (dr.if_stmt(), drjit.while_loop()) currently disable gradient tracking of all variable state when the operation takes place within an AD-disabled scope.

This can be surprising when a @dr.syntax transformation silently passes local variables to such an operation, which then become non-differentiable. @dvicini reported this in issue https://github.com/mitsuba-renderer/drjit/issues/253.

This commit carves out an exception: when variables aren't actually modified by the control flow operation, they can retain their AD identity.

The PR has 3 parts:

• the first commit changes the semantics of ad_var_inc_ref so that it only does reference counting. The old behavior (drop ref if variable is currently non-differentiable) is moved to a new function ad_var_copy_ref() (see the description of this commit for details). This already fixes 80% of the problems.
• Parts 2 and 3 fix dr.if_stmt() and dr.while_loop(), respectively.

The PR depends on a Dr.Jit-Core PR: https://github.com/mitsuba-renderer/drjit-core/pull/104

[andyyankai]

Hi, may I ask if this fix can relate to this issue? Thanks! https://github.com/mitsuba-renderer/mitsuba3/issues/1334