Port 9b3174b2

Original Commit Message:

    Remove all IsHeapObject/IsSmi checks from assembler and also from
    the macro-assembler functions that Turbofan code generation uses.

    Note for porters: In case it's unclear which macro-assembler
    functions need to be modified, it may be best to wait until I
    split MacroAssembler in a followup-CL, which will make that clear.

R=neis@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=v8:6048
LOG=N

Change-Id: Ic24c7145fa9c3d44d0359e93583eb5ddf0bcf5a8
Reviewed-on: https://chromium-review.googlesource.com/581796
Commit-Queue: Jaideep Bajwa <bjaideep@ca.ibm.com>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46845}