Instead of loading 64 bits and shifting:

      ldr x0, [x1, #offset]
      asr x0, x0, #32

    directly load the interesting 32 bits and sign-extend:

      ldrsw x0, [x1, #offset+4]

BUG=

Review-Url: https://codereview.chromium.org/2243843002
Cr-Commit-Position: refs/heads/master@{#38622}