[X86] Assertion failure in X86AsmBackend::applyFixup

https://gcc.godbolt.org/z/c9Yb9d8sf
```
.data
label1:
.byte label2 - label1
.rept 255
.byte 0
.endr
label2:
```
Compiled as:
```
llvm-mc -triple=i386 test.S -filetype=obj -o test.o
```
```
llvm-mc: /root/llvm-project/llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp:714: virtual void {anonymous}::X86AsmBackend::applyFixup(const llvm::MCFragment&, const llvm::MCFixup&, const llvm::MCValue&, uint8_t*, uint64_t, bool): Assertion `(Size == 0 || isIntN(Size * 8 + 1, SignedValue)) && "Value does not fit in the Fixup field"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
Stack dump:
0. Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/llvm-mc -filetype=obj -o example.o
#0 0x0000000000d33a18 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/llvm-mc+0xd33a18)
#1 0x0000000000d30b64 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
#2 0x0000708f01442520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#3 0x0000708f014969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#4 0x0000708f01442476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#5 0x0000708f014287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#6 0x0000708f0142871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
#7 0x0000708f01439e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#8 0x00000000009d1cac (anonymous namespace)::X86AsmBackend::applyFixup(llvm::MCFragment const&, llvm::MCFixup const&, llvm::MCValue const&, unsigned char*, unsigned long, bool) X86AsmBackend.cpp:0:0
#9 0x0000000000bbd819 llvm::MCAssembler::evaluateFixup(llvm::MCFragment const&, llvm::MCFixup&, llvm::MCValue&, unsigned long&, bool, unsigned char*) const (.constprop.0) MCAssembler.cpp:0:0
#10 0x0000000000bc2a0f llvm::MCAssembler::layout() (/opt/compiler-explorer/clang-assertions-trunk/bin/llvm-mc+0xbc2a0f)
#11 0x0000000000bc2e89 llvm::MCAssembler::Finish() (/opt/compiler-explorer/clang-assertions-trunk/bin/llvm-mc+0xbc2e89)
#12 0x0000000000be7a4c llvm::MCELFStreamer::finishImpl() (/opt/compiler-explorer/clang-assertions-trunk/bin/llvm-mc+0xbe7a4c)
#13 0x0000000000c074d7 llvm::MCStreamer::finish(llvm::SMLoc) (/opt/compiler-explorer/clang-assertions-trunk/bin/llvm-mc+0xc074d7)
#14 0x0000000000c8fa62 (anonymous namespace)::AsmParser::Run(bool, bool) AsmParser.cpp:0:0
#15 0x000000000040d714 main (/opt/compiler-explorer/clang-assertions-trunk/bin/llvm-mc+0x40d714)
#16 0x0000708f01429d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#17 0x0000708f01429e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#18 0x0000000000480115 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/llvm-mc+0x480115)
Program terminated with signal: SIGSEGV
Compiler returned: 139
```

Start with llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp at applyFixup, then run the provided llvm-mc command with the assembly reproducer. Trace how the label difference reaches the fixup assertion and verify that the input no longer aborts when the issue is resolved.

Written by the indexing model from the issue text.