This allows temporary destinations for function calls to have their
allocas omitted.

It's quite a large amount of code, and moving it into a method allowed
for some refactoring to make the logic a little easier to understand

Some types weren't being properly monomorphised, and didn't have their
regions properly erased. This is now fixed.

Also fixes an issue where a temp was initialized in two separate
branches, but wasn't given an alloca.