Hoist alloc_stack instructions of 'generic' or resilient type to the entry
block. At the same time also perform a very simple stack coloring analysis.
This does not use a true liveness-analysis yet but rather employs some simple
conservative checks to see whether the live ranges of two alloc_stacks might
interfere.
AllocStackHoisting is an IRGen SIL pass. This allows for using IRGen's type
lowering information. Furthermore, hoisting and merging the alloc_stack
instructions this late does not interfere with SIL optimizations because the
resulting SIL never gets serialized.
