Undo class_linker.cc workaround for out of registers bug.

Bug: http://b/30419309
Test: Built x86_64/x86 targets successfully with latest toolchain.

This change re-enables optimization for a function in class_linker.cc
that previously exposed an internal compiler error in Clang. Now that
the bug has been fixed, we can remove the workaround.

Change-Id: If6520c17630b6f67ce016966c0777b675be1b852