aarch64 - remove useless uxtb instructions

[swalk-cavium]

This change removes unneeded zero-extend instructions seen in various benchmarks. A less thorough version of this had been previously submitted in PR#7707.

Here is an example from hphp/test/slow/comparisons/all_type_comparison_test.php

Before

 553: CGetL L:3
    (27) t5:Bool = LdLoc<Bool,3> t0:FramePtr
        Main:
              0x27e020dc  385c03a0              ldurb w0, [x29, #-64]
    --- bc 557, fp 0, spOff 8 (test_pair)
 557: Cmp
    (37) t9:Int = CmpBool false, t5:Bool
        Main:
              0x27e020e0  52800001              movz w1, #0x0
              0x27e020e4  d3401c21              uxtb x1, w1   //<<---
              0x27e020e8  d3401c00              uxtb x0, w0   //<<---
              0x27e020ec  cb000021              sub x1, x1, x0

After

 553: CGetL L:3
    (27) t5:Bool = LdLoc<Bool,3> t0:FramePtr
        Main:
              0x31002544  385c03a0              ldurb w0, [x29, #-64]
    --- bc 557, fp 0, spOff 8 (test_pair)
 557: Cmp
    (37) t9:Int = CmpBool false, t5:Bool
        Main:
              0x31002548  aa1f03e1              mov x1, xzr
              0x3100254c  cb000021              sub x1, x1, x0

The standard regression tests were run with 6 option sets.  No new failures
were seen.

[swalk-cavium]

@mxw - Hi Max, Can you review?