[view compiler] Add conditional branch instruction

This CL adds support for the if-eqz instruction. It should be easy to add
additional comparisons as needed.

This also introduces a new kind of Value called a Label. Labels may be created
any time and then must be bound to a location in code at some point. References
to labels are tracked, and when a label is bound all references are patched to
refer to the concrete address.

Bug: 111895153
Change-Id: I15424aec75425004f0f1f4bbc6e760bac3a6c7de