Properly fold external reference access into memory operands whenever
possible, i.e. for accessing the allocation top/limit, similar to what
we do in Crankshaft and hand-written native code. This only works when
the serializer is disabled, i.e. doesn't apply to the stubs in the
snapshot (for now). This reduces register pressure especially around
allocations where we'd currently need two registers to hold both the
allocation top and limit pointers in registers (on x64).

R=epertoso@chromium.org

Review-Url: https://codereview.chromium.org/2398603002
Cr-Commit-Position: refs/heads/master@{#39993}