Optimizing: Simplify consecutive type conversions.

Merge two consecutive type conversions to one if the result
of such merged conversion is guaranteed to be the same and
remove all implicit conversions, not just conversions to the
same type. Improve codegens to handle conversions from long
to integral types smaller than int.

This will make it easier to simplify `(byte) (x & 0xffL)` to
`(byte) x` where the conversion from long to byte is done by
two dex instructions, long-to-int and in int-to-byte.

Bug: 23965701
Change-Id: I833f193556671136ad2cd3f5b31cdfbc2d99c19d