Animation tree blending bug

[ICatRegister]

Godot version:
3.2.3 stable

OS/device including version:
Windows 10

Issue description:
Setting the parameter to a negative value on the Blend3 node leads to jerking when switching animations if blending is enabled between these animations.
When using the Add3 node, it's even worse: the effect is the same, but the animation does not work with a negative parameter

Steps to reproduce:

• Download the project.
• Set the parameter on the Blend3 node to a negative value.
• Switch between animations 1 and 2.

With a positive parameter on Blend3 or zero xfade_time on Transition node, the effect does not appear

Minimal reproduction project:

AnimTreeBlendBug.zip

[Calinou]

Duplicate of #37661 and possibly #23414.