optimizer: fix warning in merge_types in 32 bits

I get these warnings in Visual Studio Express (32 bits):

..\..\racket\src\optimize.c(3738): warning C4244: '=' : conversion from 'mzlonglong' to 'intptr_t', possible loss of data
..\..\racket\src\optimize.c(3742): warning C4244: '=' : conversion from 'mzlonglong' to 'intptr_t', possible loss of data