When working on csarofeen/pytorch#1900, I find
that sometimes expr simplifier will assign the dtype of a value wrongly.
This is because expr simplifier is written loosely assuming the dtypes
of all `Val`s are the same. However, because we are putting pointer
types into the expression, we need to be more careful, otherwise we will
get wrong kernel code like:
```
__half* ptr1 = threadIdx.x * 128;
__half* ptr2 = ptr1 + 256 + T1.data;
```
This PR changes all passes in the expr simplifier to let it infer dtype
from its inputs.

- [x] TODO: update all the failing `assertCUDAKernel` tests😵‍💫😵‍💫😵‍💫😵‍💫