Rewrite GVN's field id and field type handling.

Rewrite GVN's field id and field type handling.

Create a helper unit for dex insn classification and cache
dex field type (as encoded in the insn) in the MirFieldInfo.
Use this for cleanup and a few additional DCHECKs.

Change the GVN's field id to match the field lowering info
index (MIR::meta::{i,s}field_lowering_info), except where
multiple indexes refer to the same field and we use the
lowest of the applicable indexes. Use the MirMethodInfo from
MIRGraph to retrieve field type for GVN using this index.
This slightly reduces GVN compilation time and prepares for
further compilation time improvements.

Change-Id: I1b1247cdb8e8b6897254e2180f3230f10159bed5