ARM64: Use the shifter operand to merge in previous shift instructions.

When possible, we transform sequences of code of the form
    lsl x8, x9, #imm
    add x0, x1, x8
into
    add x0, x1, x9 LSL #imm

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/257203002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21161 ce2b1a6d-e550-0410-aec6-3dcde31c8c00