[lld] [lld][ELF][LoongArch] Support relaxing R_LARCH_CALL36 (PR #127312)

[WÁNG Xuěruì via llvm-commits]

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@@ -830,6 +833,45 @@ static void relaxPCHi20Lo12(Ctx &ctx, const InputSection &sec, size_t i,
   remove = 4;
 }
 
+static bool isInsnPairCall36(uint64_t pair) {
+  const uint32_t insn1 = extractBits(pair, 31, 0);
+  const uint32_t insn2 = extractBits(pair, 63, 32);
+  if ((insn1 & 0xfe000000) != PCADDU18I)
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xen0n wrote:

`ld.bfd` checks for `jirl` but not `pcaddu18i`, nor does `ld.bfd` ensure `jirl.rj == pcaddu18i.rd`. Granted, these are irrelevant for well-formed inputs, but I think such checks should get added to `ld.bfd` (or indeed, the psABI) as well, for the guarantee that "relaxation does not affect code semantics" to uphold.

https://github.com/llvm/llvm-project/pull/127312