[BPF] Emit fatal error if out of range for FK_PCRel_2 branch target

Currently for the branch insn like
   "if $dst "#OpcodeStr#" $imm goto $BrDst"
The $BrDst range needs to be in the range of [INT16_MIN, INT16_MAX].

When running bpf selftest with latest llvm, I found
pyperf600.o generated insn with range outside
of [INT16_MIN, INT16_MAX], which caused verifier failure.
See below insn rust-lang#12.

  0000000000000000 <on_event>:
  ; {
         0:       7b 1a 00 ff 00 00 00 00 *(u64 *)(r10 - 256) = r1
  ;       uint64_t pid_tgid = bpf_get_current_pid_tgid();
         1:       85 00 00 00 0e 00 00 00 call 14
         2:       bf 06 00 00 00 00 00 00 r6 = r0
  ;       pid_t pid = (pid_t)(pid_tgid >> 32);
         3:       bf 61 00 00 00 00 00 00 r1 = r6
         4:       77 01 00 00 20 00 00 00 r1 >>= 32
         5:       63 1a fc ff 00 00 00 00 *(u32 *)(r10 - 4) = r1
         6:       bf a2 00 00 00 00 00 00 r2 = r10
         7:       07 02 00 00 fc ff ff ff r2 += -4
  ;       PidData* pidData = bpf_map_lookup_elem(&pidmap, &pid);
         8:       18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
        10:       85 00 00 00 01 00 00 00 call 1
        11:       bf 08 00 00 00 00 00 00 r8 = r0
  ;       if (!pidData)
        12:       15 08 15 e8 00 00 00 00 if r8 == 0 goto -6123 <LBB0_27588+0xffffffffffdae100>
        13:       b4 01 00 00 00 00 00 00 w1 = 0

We may need to add new insn to extend the range of $BrDst.
This patch added a fatal error if out of range so compiler can warn
the otherwise incorrect code generation.

Differential Revision: https://reviews.llvm.org/D123877

llvm/lib/Target/BPF/MCTargetDesc/BPFAsmBackend.cpp

@@ -87,6 +87,11 @@ void BPFAsmBackend::applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
     }
   } else {
     assert(Fixup.getKind() == FK_PCRel_2);
+
+    int64_t ByteOff = (int64_t)Value - 8;
+    if (ByteOff > INT16_MAX * 8 || ByteOff < INT16_MIN * 8)
+      report_fatal_error("Branch target out of insn range");
+
     Value = (uint16_t)((Value - 8) / 8);
     support::endian::write<uint16_t>(&Data[Fixup.getOffset() + 2], Value,
                                      Endian);