Use native_int instead of int64/int32 in IR test output

[JukkaL]

Copy-pasting IR build test output often results in CI failures, since the output may contain types like int64, which are platform-dependent -- and should be native_int. This causes the 32-bit Windows build to fail.

native_int seems to be almost always what we want, so I'd suggest replacing int64/int32 in test output with native_int. This will also make it easier to bulk update test cases if there are changes in the generated IR.

So instead of having to manually replace int64 with native_int, we'll sometimes need to replace native_int with int64. This seems like a good trade-off.

[TH3CHARLie]

Raising priority as more and more mypyc contributors are facing this issue.