[lworld] C1 assertion failure in LIRAssembler::patching_epilog

Test compiler/valhalla/inlinetypes/TestLWorld.java fails with the following assertion failure:

# Internal Error (/oracle/valhalla/open/src/hotspot/share/c1/c1_LIRAssembler.cpp:73), pid=1985987, tid=1986004
# Error: ShouldNotReachHere()
#

This is due to incorrect changes from JDK-8327150 leading to the generation of patchable code in the access to a flat field, which is not expected to happen.

Another issue discovered during the investigation of this problem is the removal of an optimization related to empty value classes. Another assert can fail because of the creation of a new instance of an empty value class, when such creation should be avoided.