aarch64 - sxtb/lsl peephole

[swalk-cavium]

This adds a peephole which captures a common pattern seen in Mediawiki traces.

Before

0xcb00a04  93401c21     sxtb x1, w1
0xcb00a08  d37ef421     lsl x1, x1, #2

After

0xcb00a04  937e1c21     sbfiz  x1, x1, #2,  #8

The standard regression tests were run with 6 option sets. No new issues were seen. The mediawiki oss-performance test was run without incident.

[swalk-cavium]

@mxw - Can you inspect?