Improve HLoadClass code generation.

For classes in the boot image, use either direct pointers
or PC-relative addresses. For other classes, use PC-relative
access to the dex cache arrays for AOT and direct address of
the type's dex cache slot for JIT.

For aosp_flounder-userdebug:
  - 32-bit boot.oat: -252KiB (-0.3%)
  - 64-bit boot.oat: -412KiB (-0.4%)
  - 32-bit dalvik cache total: -392KiB (-0.4%)
  - 64-bit dalvik-cache total: -2312KiB (-1.0%)
    (contains more files than the 32-bit dalvik cache)
For aosp_flounder-userdebug forced to compile PIC:
  - 32-bit boot.oat: -124KiB (-0.2%)
  - 64-bit boot.oat: -420KiB (-0.5%)
  - 32-bit dalvik cache total: -136KiB (-0.1%)
  - 64-bit dalvik-cache total: -1136KiB (-0.5%)
    (contains more files than the 32-bit dalvik cache)

Bug: 27950288
Change-Id: I4da991a4b7e53c63c92558b97923d18092acf139