Improve the S390 allocate sequence by:
- Keeping allocation limit in memory, and leverage compare RX-instr.
- Prefetching subsequent cache lines from allocation top.
- Optimizing object tagging with LA
- Optimizing increment for Fast-Allocate with ASI/AGSI

R=jyan@ca.ibm.com, michael_dawson@ca.ibm.com, bjaideep@ca.ibm.com
BUG=

Review-Url: https://codereview.chromium.org/2601563002
Cr-Commit-Position: refs/heads/master@{#41933}