Currently, the LiftoffCompiler can bail out on unsupported
instructions, but the LiftoffAssembler can not. This makes it difficult
to iteratively implement the platform specific part of the assembler.
With this CL, also the assembler can bail out if an unimplemented
assembler method is called. This allows to test already implemented
methods.

R=ahaas@chromium.org
CC=sreten.kovacevic@mips.com

Bug: v8:6600
Change-Id: Ieb7abc2188266bb93c40fe55fd6ee0e5b1e0d220
Reviewed-on: https://chromium-review.googlesource.com/909390
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51242}