[VPlan] Create edge masks all cases up front needed.(NFC)

Similarly to how block masks are created up front and later only
retrieved also make sure masks are created in cases where edge masks are
needed, i.e. blend recipes.

Creating block-in masks for all blocks in the loop also ensures edge
masks for all relevant edges have been created. Later, the new
getEdgeMask can be used to look up cached edge masks.

This makes sure edge masks are available in all cases for
https://github.com/llvm/llvm-project/pull/76090.