MIPS: Consistenly handle power-of-2 divisors in division-like operations.

MIPS: Consistenly handle power-of-2 divisors in division-like operations.

Port r19715 (0d6d244)

Original commit message:
Lithium currently supports 3 division-like operations on integral
operands: "Normal" division (rounding towards zero), flooring division
(rounding towards -Infinity) and modulus calculation (the counterpart
for the "normal" division). For divisors which are a power of 2, one can
efficiently use some bit fiddling to avoid the actual division for such
operations. This CL cleanly splits off these operations into separate
Lithium instructions, making the code much more maintainable and more
consistent across platforms.

There are 2 basic variations of these bit fiddling algorithms: One
involving branches and a seemingly more clever one without branches.
Choosing between the two is not as easy as it seems: Benchmarks (and
probably real-world) programs seem to favor positive dividends,
registers and shifting units are sometimes scarce resources, and branch
prediction is quite good in modern processors. Therefore only the
"normal" division by a power of 2 is implemented in a branch-free
manner, this seems to be the best approach in practice. If this turns
out to be wrong, we can easily and locally change this.

BUG=

[Paul Lind, 2014-03-07 22:09:53]

LGTM. Note to Sven and rest of Google team, this gets us building/testing ok,
but is an interim step. We'll revisit this next week, to bring the mips division
support more in line with the other arch's.

[Paul Lind, 2014-03-07 22:15:30]

Committed as r19733.