Consistently use a separate Lithium instruction for flooring division.

Previously we tried to share some code on by a slightly confusing re-use
of LDivI for a (general) flooring division. Now we cleanly separate
concerns, just like for the rest of the division-like operations. Note
that ARM64 already did it this way.

If we really want to save some code, we can introduce some macro
assembler instructions and/or helper functions in the code generator in
a future CL, but we should really try to avoid being "clever" to save
just a few lines of trivial code. Effort != complexity. :-)

Renamed some related Lithium operands on the way for more consistency.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/212703002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20395 ce2b1a6d-e550-0410-aec6-3dcde31c8c00