[AArch64] Remove an unnecessary cast (NFC) (#155134)

Val is already of int64_t.

Author: kazutakahirata

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -18309,7 +18309,7 @@ bool AArch64TargetLowering::shouldConvertConstantLoadToIntImm(const APInt &Imm,
   if (Val == 0 || AArch64_AM::isLogicalImmediate(Val, BitSize))
     return true;
 
-  if ((int64_t)Val < 0)
+  if (Val < 0)
     Val = ~Val;
   if (BitSize == 32)
     Val &= (1LL << 32) - 1;