Build stack allocations at the function entry if possible

[xermicus]

Stack allocations built after the first basic block can hurt some performance optimizations. Hypothetically, it should be safe to always insert them there. Practically however it depends on where and how stack allocations are used throughout the translation. Stack allocations can safely be moved to the entry block of our functions iff the program semantics are preserved; which depends on control flow and requires the allocation size to be static.

We should decide for each instance of build_alloca whether it is safe to build them at the function entry and, if appropriate, change them to the fn build_alloca_at_entry helper instead.