Treat some right shifts as narrowing conversions.

Treat some right shifts as narrowing conversions.

Java bytecode optimizers like ProGuard will remove explicit integer
width conversions from certain sequences.  For example, after
right-shifting an integer 24 times, an int-to-byte instruction is
redundant.

This change teaches the verifier that right shifts sometimes reduce
(or at least don't increase) the width of an integer.  Previously, the
result of a right shift was always a full-sized int.