As @AndroidEx correctly stated, assignments are not expressions in Kotlin, unlike Java. The reason is that expressions with side effects are generally discouraged. See this discussion on a similar topic.
One solution is just to split the expression and move the assignment out of condition block:
a = b
if (a != c) { ... }
Another one is to use functions from stdlib like let, which executes the lambda with the receiver as parameter and returns the lambda result. apply and run have similar semantics.
if (b.let { a = it; it != c }) { ... }
if (run { a = b; b != c }) { ... }
Thanks to inlining, this will be as efficient as plain code taken from the lambda.
Your example with InputStream would look like
while (input.read(bytes).let { tmp = it; it != -1 }) { ... }
Also, consider readBytes function for reading a ByteArray from an InputStream.