PPC: Reshuffle registers in JSConstructStub to avoid trashing costructor and new.target on fast path (so we don't need to push/pop them).

Port 0ef5ad5a

Original commit message:
    This CL also fixed register usages in MacroAssembler::Allocate() broken by 2fc2cb99 (r32144).

R=ishell@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=chromium:560239
LOG=Y

Review URL: https://codereview.chromium.org/1472823004

Cr-Commit-Position: refs/heads/master@{#32230}