ARM64: Move sign-extension to load instructions.

The SXTW extend mode is usually free when used in a load or store
operand, but it can slightly increase the latency of an arithmetic
instruction. This patch optimises some address calculations accordingly:
  - PrepareKeyedExternalArrayOperand.
  - BuildSeqStringOperand.
  - DoStringCharFromCode.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/247443002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20891 ce2b1a6d-e550-0410-aec6-3dcde31c8c00