Subzero: Speed up VariablesMetadata initialization.

Subzero: Speed up VariablesMetadata initialization.

Currently, O2 calls VariablesMetadata::init() 4 times:

- Twice for liveness analysis, where only multi-block use information is needed for dealing with sparse bit vectors.

- Once for address mode inference, where single-definition information is needed.

- Once for register allocation, where all information is needed, including the set of all definitions which is needed for determining AllowOverlap.

So we limit the amount of data we gather based on the actual need.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/650613003