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.

Bug: v8:6048
Change-Id: Ife0735cc6f48713c9ec493faf2dac5e553d1c06b
Reviewed-on: https://chromium-review.googlesource.com/561015
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46436}