MIPS: Change CmpWeakValue to a more MIPS like GetWeakValue.

This approach saves one instruction by eliminating the use of Subu as a
pseudo-comparison.

BUG=

Review URL: https://codereview.chromium.org/779793004

Cr-Commit-Position: refs/heads/master@{#25662}
Explicitly track whether incremental marking was activated

In the gc-tracer, we check whether we're marking to figure out which
part of the mark compact we're in. If we aborted incremental marking for
whatever reason, the check fails and we might later run into trouble

BUG=none
R=hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/783453003

Cr-Commit-Position: refs/heads/master@{#25663}
MIPS; Improve checked load/store operators for constant offset and length.

TEST=
BUG=
R=balazs.kilvady@imgtec.com, paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/782493002

Cr-Commit-Position: refs/heads/master@{#25664}