For the HeapNumber case, use Float64Neg directly instead of a
Float64Mul by -1.0.

For the Smi case, logic is added to handle the boundary conditions
(0 and Smi::kMinValue), and the general case is handled by a SmiSub
from 0.

Change-Id: I110916d9d1eb5d22d618fbf358d8d5b63cc71b3a
Reviewed-on: https://chromium-review.googlesource.com/663945Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47995}