C2 compilation fails with "duplicated predicate failed which is impossible"

This was found by fuzzing. This is the first time I see anything like this, and there are no relevant hits in the bugtracker, so I have to assume this is new.

Reproduces reliably with the attached fuzzer case.

# Internal Error (/home/shade/trunks/jdk-jdk/src/hotspot/cpu/x86/macroAssembler_x86.cpp:892), pid=1145, tid=1146
# fatal error: DEBUG MESSAGE: duplicated predicate failed which is impossible
#
# JRE version: OpenJDK Runtime Environment (14.0) (fastdebug build 14-internal+0-adhoc.shade.jdk-jdk)
# Java VM: OpenJDK 64-Bit Server VM (fastdebug 14-internal+0-adhoc.shade.jdk-jdk, mixed mode, sharing, tiered, compressed oops, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x1244621] MacroAssembler::debug64(char*, long, long*)+0x41

--------------- T H R E A D ---------------

Current thread (0x00007f098c024000): JavaThread "main" [_thread_in_Java, id=1146, stack(0x00007f0994a3f000,0x00007f0994b40000)]

Stack: [0x00007f0994a3f000,0x00007f0994b40000], sp=0x00007f0994b3e630, free space=1021k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native c