PPC instr has 16bits to represent an imm, load constant
in register if imm is not in the range [-2^15, 2^15)

R=joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
Log=N

Bug: 
Change-Id: Id9aa97538b1f93f01d5a297b6256e1b082f06ca1
Reviewed-on: https://chromium-review.googlesource.com/624714Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Commit-Queue: Jaideep Bajwa <bjaideep@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#47487}