The idea of GetUnaryOpTargetRegister and GetBinaryOpTargetRegister is
to reuse the register of one of the operands instead of using an
separate third one, because we can often generate better code if the
destination register matches the src or lhs.
This was implemented by looking at the top or the first two stack
entries, and using one of their registers if there is only one use.
Instead of doing that we can also just pop them and then later check
whether this was the only use. This makes the code smaller, more
readable and probably faster.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ia5d9e320bdb3add5032400455a64a0c7fee77cbd
Reviewed-on: https://chromium-review.googlesource.com/950947Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51770}