ARM64: Fix LCodeGen::ToOperand32.

This fixes the following generated code sequence:
  movn w1, #0     // Synthesize -1.
  cmp w0, w1

With a properly-constructed Operand, the MacroAssembler can optimize it
as follows:
  cmn w0, #1

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/253513003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20989 ce2b1a6d-e550-0410-aec6-3dcde31c8c00