Drop the typing rules for the machine operators and replace them
with UNREACHABLE. These typing rules were never correct and there's
also no need to have those rules at all.

Drive-by-fix: Remove the extremely annoying test-simplified-lowering.cc
file, which is not very useful, but consumes a large amount of time to
keep it compiling and passing. Instead we should introduce appropriate
tests for the SimplifiedLowering that also test something meaningful
w/o just cementing the implementation.

R=jarin@chromium.org
BUG=v8:5267

Review-Url: https://codereview.chromium.org/2292463002
Cr-Commit-Position: refs/heads/master@{#38970}