Now that the GC team has landed the appropriate changes to ensure that the top
page of the address space is never used for allocation, the inlined fast-case
allocation path in the CodeAssembler can be micro-optimized to an add to top
followed by an unsigned compare to limit, eliding a no-longer-needed overflow
check.

Review-Url: https://codereview.chromium.org/1923803003
Cr-Commit-Position: refs/heads/master@{#35830}