Rewrite null checks in read barrier introspection thunks.

Rely on the implicit null check in the fast path.

Test: Manual; run-test --gdb 160, break in the introspection
      entrypoint, find the mf.testField0000 read barrier
      code in the caller (this one has a stack map for null
      check, most other reads do not need one), break there,
      step into the thunk, overwrite the base register with
      0 and observe the NPE being thrown. Repeat with --64.
Test: Pixel 2 XL boots.
Test: testrunner.py --target --optimizing
Bug: 36141117
Change-Id: I61f879f22f5697a4108f1021eb0e3add742c8755