[turbofan]: Use "leal" more prevasively on x64

Only use "addl" and "subl" in cases that have been measured to be
faster (currently only immediate operations).

Review URL: https://codereview.chromium.org/735293004

Cr-Commit-Position: refs/heads/master@{#25580}
Use weak cells in map checks in polymorphic ICs.

BUG=v8:3663
LOG=N

Review URL: https://codereview.chromium.org/753993003

Cr-Commit-Position: refs/heads/master@{#25581}
[x64] introduce vex prefix version of float64 arithmetic binop

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/764863002

Patch from Weiliang Lin <weiliang.lin@intel.com>.

Cr-Commit-Position: refs/heads/master@{#25582}