In fast-allocate, the path that leverages Add Mem-Imm fails to take
into account that the allocation size may be adjusted by kDoubleSize/2
for alignment.  Limit this instruction to 64-bit only.

Also guard PFDs with the proper facility check.
R=jyan@ca.ibm.com, michael_dawson@ca.ibm.com, bjaideep@ca.ibm.com
BUG=

Review-Url: https://codereview.chromium.org/2605063002
Cr-Commit-Position: refs/heads/master@{#41978}