This patch updates some places where VectorLoopValueMap is accessed
directly to instead go through VPTransformState.
As we move towards managing created values exclusively in VPTransformState,
this ensures the use always can fetch the correct value.
This is in preparation for D92285, which switches to managing scalarized
values through VPValues.
In the future, the various fix* functions should be moved directly into
the VPlan codegen stage.