Optimizing: Refactor and speed up LinearScan.

Move the linear scan code to an inner helper class that
holds data needed by the current pass (core registers or
FP registers) to avoid frequent checks of the current
register type.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 181943478
Flag: EXEMPT refactor
Change-Id: Ib81a93d0c9ccc20df1b0bcca84a906d3aef8abf5