fix(asm): encode the arm64 bitfield aliases with their own opc

Assisted-by: GLM 5.3 Flash

asm/arm64_assemble.go

@@ -3636,7 +3636,10 @@ func encodeARM64BitfieldAlias(mnem string, baseOp uint32, ops []*ast.Operand) ([
 	case "BFXIL", "BFXILW":
 		immr, imms = lsb, lsb+width-1
 	default: // BFI, SBFIZ, UBFIZ
-		immr, imms = (-lsb)%bits, width-1
+		// immr = (bits - lsb) mod bits, the toolchain's 64-r form with the
+		// zero lsb folding to zero (Go's % keeps the sign, so the operand
+		// order matters here).
+		immr, imms = (bits-lsb)%bits, width-1
 	}
 	return a64wordLE(baseOp | uint32(immr)<<16 | uint32(imms)<<10 | uint32(rn)<<5 | uint32(rd)), nil
 }