Now that all accesses to the last match info are in C++ and TF code, we can
finally turn the last match info into a FixedArray. Similar to the ArrayList,
it uses its first field to store its length and grows dynamically in amortized
O(1) time.

Unlike previously, this means that the last match info pointer stored on the
context can actually change (in case the FixedArray needs to grow).

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2415103002
Cr-Commit-Position: refs/heads/master@{#40308}