IMPORT: slz: avoid multiple shifts on 64-bits

IMPORT: slz: avoid multiple shifts on 64-bits

On 64-bit platforms, disassembling the code shows that send_huff() performs
a left shift followed by a right one, which are the result of integer
truncation and zero-extension caused solely by using different types at
different levels in the call chain. By making encode24() take a 64-bit
int on input and send_huff() take one optionally, we can remove one shift
in the hot path and gain 1% performance without affecting other platforms.

This is slz upstream commit fd165b36c4621579c5305cf3bb3a7f5410d3720b.