AArch64: I2L/L2I conversions can be skipped for masked positive values

On the following example sxtw (i2l) can be skipped because the value is positive.

; - org.apache.lucene.util.packed.DirectReader$DirectPackedReader16::get@12 (line 185)
0x0000ffff706b7d54: sxth w10, w10
0x0000ffff706b7d58: and w10, w10, #0xffff
0x0000ffff706b7d5c: sxtw x2, w10 ; this i2l is excessive

https://github.com/apache/lucene-solr/blob/master/lucene/core/src/java/org/apache/lucene/util/packed/DirectReader.java#L185
183 public long get(long index) {
184 try {
185 return in.readShort(offset + (index << 1)) & 0xFFFF;