PPC: [turbofan] Load immortal heap objects from the heap roots.

Port 5d2de78a

Original commit message:
It's cheaper to materialize heap constants by loading from the roots
array instead of embedding the constant into the instruction stream, at
least on x64, arm and arm64.

Drive-by-fix: Also cleanup the materialize constant from frame
optimization.

R=michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1075303003

Cr-Commit-Position: refs/heads/master@{#27823}