Fix Yul renaming woes - Alloc before Assign

[mbenke]

Extension of the fix for #188 done in #326: declare/allocate the variable before assigning to it. The problem was e.g. with

 function copyToMem(reader:MemoryWordReader, dst:word, cnt: word) -> () {
      match reader {
      | MemoryWordReader(ptr) => assembly { mcopy(dst, ptr, cnt) }
      }
  }

where match for one-constructor types is eliminated and the body compiles to just asm:

function copyToMem(reader:MemoryWordReader, dst:word, cnt: word) -> () {
  assembly { mcopy(dst, ptr, cnt) }
}

...but now ptr is undefined. The fix in #326 in this case adds an assignment ptr = reader (on MAST level, so there is no type mismatch issue). This PR adds the missing declaration (or actually an allocation for ptr)

[rodrigogribeiro]

LGTM